Observable: Difference between revisions

From formulasearchengine
Jump to navigation Jump to search
en>Samkohn
No edit summary
 
en>Jrgetsin
→‎Further reading: added citation to Weyl
Line 1: Line 1:
If you compare registry cleaners there are a number of points to look out for. Because of the sheer number of for registry cleaners available found on the Internet at the moment it can be very convenient to be scammed. Something often overlooked is that a few of these products will after all end up damaging your PC. And the registry they say they have cleaned will just lead to more difficulties with a computer than the ones you began with.<br><br>Before actually obtaining the software it happens to be best to check found on the firms which create the software. If you may discover details found on the form of standing every organization has, perhaps the risk of malicious programs can be reduced. Software from reputed companies have helped me, and several other users, to make my PC run quicker.. If the product description does not look wise to we, does not include details about the software, refuses to include the scan functions, we should go for another one which ensures you're paying for what we need.<br><br>Over time a disk can additionally get fragmented. Fragmentation causes the computer to slow down considering it takes windows much longer to find a files location. Fortunately, the PC has a built in disk defragmenter. We can run this system by clicking "Start" - "All Programs" - "Accessories" - "System Tools" - "Disk Defragmenter". You usually today have the option to choose which drives or partition we want to defragment. This action may take we certain time so it is actually advised to do this on a regular basis thus because to avoid further fragmentation plus to accelerate the windows XP computer.<br><br>Chrome allows customizing itself by applying variety of themes accessible online. If you had newly used a theme which no longer functions correctly, it results in Chrome crash on Windows 7. It is recommended to set the original theme.<br><br>The [http://bestregistrycleanerfix.com/tune-up-utilities tuneup utilities 2014] could come because standard with a back up and restore facility. This ought to be an easy to apply task.That signifies that should you encounter a problem with a PC following utilizing a registry cleaning we can simply restore the settings.<br><br>Why this issue occurs frequently? What are the causes of it? In fact, there are 3 major causes that may cause the PC freezing problem. To resolve the issue, you require to take 3 steps in the following paragraphs.<br><br>The 'registry' is merely the central database which stores all a settings and choices. It's a truly significant part of the XP system, meaning which Windows is consistently adding plus updating the files inside it. The problems occur whenever Windows really corrupts & loses several of these files. This makes a computer run slow, because it attempts hard to locate them again.<br><br>Registry products will aid your computer run inside a better mode. Registry products should be piece of the usual scheduled repair program for your computer. You don't have to wait forever for the computer or the programs to load and run. A small repair may bring back the speed you lost.
In [[mathematics]], in particular in [[algebraic topology]], [[differential geometry and topology|differential geometry]] and [[algebraic geometry]], the '''Chern classes''' are  [[characteristic class]]es associated to complex [[vector bundle]]s.
 
Chern classes were introduced by {{harvs|txt|authorlink=Shiing-Shen Chern|first=Shiing-Shen|last=Chern|year=1946}}.
 
== Geometric approach ==
=== Basic idea and motivation ===
 
Chern classes are [[characteristic class]]es. They are [[topological invariant]]s associated to vector bundles on a smooth manifold. The question of whether two ostensibly different vector bundles are the same can be quite hard to answer.  The Chern classes provide a simple test: if the Chern classes of a pair of vector bundles do not agree, then the vector bundles are different. The converse, however, is not true.
 
In topology, differential geometry, and algebraic geometry, it is often important to count how many [[linearly independent]] sections a vector bundle has. The Chern classes offer some information about this through, for instance, the [[Riemann-Roch theorem]] and the [[Atiyah-Singer index theorem]].
 
Chern classes are also feasible to calculate in practice. In differential geometry (and some types of algebraic geometry), the Chern classes can be expressed as polynomials in the coefficients of the [[curvature form]].
 
=== Construction of Chern classes ===
 
There are various ways of approaching the subject, each of which focuses on a slightly different flavor of Chern class.
 
The original approach to Chern classes was via algebraic topology: the Chern classes arise via [[homotopy theory]] which provides a mapping associated to ''V'' to a [[classifying space]] (an infinite [[Grassmannian]] in this case). Any vector bundle ''V'' over a manifold may be realized as the pullback of a universal bundle over the classifying space, and the Chern classes of ''V'' can therefore be defined as the pullback of the Chern classes of the universal bundle; these universal Chern classes in turn can be explicitly written down in terms of [[Schubert cycle]]s.
 
Chern's approach used differential geometry, via the curvature approach described predominantly in this article. He showed that the earlier definition was in fact equivalent to his.
 
There is also an approach of [[Alexander Grothendieck]] showing that axiomatically one need only define the line bundle case.
 
Chern classes arise naturally in [[algebraic geometry]].  The generalized Chern classes in algebraic geometry can be defined for vector bundles (or more precisely, [[locally free sheaves]]) over any nonsingular variety.  Algebro-geometric Chern classes do not require the underlying field to have any special properties.  In particular, the vector bundles need not necessarily be complex.
 
Regardless of the particular paradigm, the intuitive meaning of the Chern class concerns 'required zeroes' of a [[Section (category theory)|section]] of a vector bundle: for example the theorem saying one can't comb a hairy ball flat ([[hairy ball theorem]]). Although that is strictly speaking a question about a ''real'' vector bundle (the "hairs" on a ball are actually copies of the real line), there are generalizations in which the hairs are complex (see the example of the complex hairy ball theorem below), or for 1-dimensional projective spaces over many other fields.
 
See [[Chern-Simons]] for more discussion.
 
== The Chern class of line bundles ==
 
{{for|a sheaf theoretic description|Exponential sheaf sequence}}
 
An important special case occurs when ''V'' is a [[line bundle]]. Then the only nontrivial Chern class is the first Chern class, which is an element of the second cohomology group of ''X''. As it is the top Chern class, it equals the [[Euler class]] of the bundle.
 
The first Chern class turns out to be a [[complete invariant]] with which to classify complex line bundles, topologically speaking. That is, there is a [[bijection]] between the isomorphism classes of line bundles over ''X'' and the elements of ''H''<sup>2</sup>(''X'';''Z''), which associates to a line bundle its first Chern class. Addition in the second dimensional cohomology group coincides with [[tensor product]] of complex line bundles. {{Citation needed|date=April 2011}}
 
In algebraic geometry, this classification of (isomorphism classes of) complex line bundles by the first Chern class is a crude approximation to the classification of (isomorphism classes of) holomorphic line bundles by [[linear equivalence]] classes of [[divisor]]s.
 
For complex vector bundles of dimension greater than one, the Chern classes are not a complete invariant.
 
==The Chern class in [[Chern-Weil theory]]==
 
===The Chern class of a Hermitian vector bundle on a smooth manifold===
Given a complex [[Hermitian metric|hermitian]] [[vector bundle]] ''V'' of [[vector bundle|complex rank]] ''n'' over a [[smooth manifold]] ''M'', a representative of each Chern class (also called a '''Chern form''') ''c<sub>k</sub>''(''V'') of ''V'' are given as the coefficients of the [[characteristic polynomial]] of the [[curvature form]] Ω of ''V''.
 
:<math>\det \left(\frac {it\Omega}{2\pi} +I\right) = \sum_k c_k(V) t^k</math>
 
The determinant is over the ring of ''n'' × ''n'' matrices whose entries are polynomials in ''t'' with coefficients in the commutative algebra of even complex differential forms on ''M''.  The [[curvature form]] Ω of ''V'' is defined as
 
: <math>\Omega=d\omega+\tfrac{1}{2}[\omega,\omega]</math>
 
with ω the [[connection form]]  and ''d'' the [[exterior derivative]], or via the same expression in which ω is a [[gauge form]] for the [[gauge group]] of ''V''. The scalar ''t'' is used here only as an [[indeterminate (variable)|indeterminate]] to [[generating function|generate]] the sum from the determinant, and ''I'' denotes the ''n'' × ''n'' [[identity matrix]].
 
To say that the expression given is a ''representative'' of the Chern class indicates that 'class' here means [[up to]] addition of an [[exact differential form]]. That is, Chern classes are [[cohomology class]]es in the sense of [[de Rham cohomology]]. It can be shown that the cohomology class of the Chern forms do not depend on the choice of connection in ''V''.
 
Using the matrix identity tr(ln(''X''))=ln(det(''X'')) and the Maclaurin series for ln(''X''+''I''), this expression for the Chern form expands as
 
:<math>\sum_k c_k(V) t^k = \left[ I
      + i \frac{\mathrm{tr}(\Omega)}{2\pi} t
      +  \frac{\mathrm{tr}(\Omega^2)-\mathrm{tr}(\Omega)^2}{8\pi^2} t^2
      + i \frac{-2\mathrm{tr}(\Omega^3)+3\mathrm{tr}(\Omega^2)\mathrm{tr}(\Omega)-\mathrm{tr}(\Omega)^3}{48\pi^3} t^3
      + \cdots
      \right].</math>
 
===Example:  the complex tangent bundle of the Riemann sphere===
 
Let '''CP'''<sup>1</sup> be the [[Riemann sphere]]: 1-dimensional [[complex projective space]]. Suppose that ''z'' is a [[holomorphic]] [[manifold|local coordinate]] for the Riemann sphere. Let ''V'' = ''T'''''CP'''<sup>1</sup> be the bundle of complex tangent vectors having the form ''a''∂/∂''z'' at each point, where ''a'' is a complex number.  We prove the complex version of the ''[[hairy ball theorem]]'':  ''V'' has no section which is everywhere nonzero.
 
For this, we need the following fact: the first Chern class of a trivial bundle is zero, i.e.,
 
: <math>c_1({\mathbf C\mathbf P}^1\times {\mathbf C})=0.</math>
 
This is evinced by the fact that a trivial bundle always admits a flat connection.
 
So, we shall show that
 
:<math>c_1(V) \not= 0.</math>
 
Consider the [[Kähler metric]]
 
:<math>h = \frac{dzd\bar{z}}{(1+|z|^2)^2}.</math>
 
One readily shows that the curvature 2-form is given by
 
:<math>\Omega=\frac{2dz\wedge d\bar{z}}{(1+|z|^2)^2}.</math>
 
Furthermore, by the definition of the first Chern class
 
:<math>c_1= \left[\frac{i}{2\pi} \mathrm{tr} \ \Omega\right] .</math>
 
We must show that this cohomology class is non-zero. It suffices to compute its integral over the Riemann sphere:
 
:<math>\int c_1 dz\wedge d\bar{z} =\frac{i}{\pi}\int \frac{dz\wedge d\bar{z}}{(1+|z|^2)^2}=2</math>
 
after switching to [[polar coordinates]]. By [[Stokes' theorem]], an [[exact form]] would integrate to 0, so the cohomology class is nonzero.
 
This proves that ''T'''''CP'''<sup>1</sup> is not a trivial vector bundle.
 
== Properties of Chern classes ==
 
Given a complex [[vector bundle]] ''V'' over a [[topological space]] ''X'', the Chern classes of ''V'' are a sequence of elements of the [[cohomology]] of ''X''. The '''''k''-th Chern class''' of ''V'', which is usually denoted ''c<sub>k</sub>''(''V''), is an element of
 
:''H''<sup>2''k''</sup>(''X'';'''Z'''),
 
the cohomology of ''X'' with [[integer]] coefficients.  One can also define the '''total Chern class'''
 
:<math>c(V) = c_0(V) + c_1(V) + c_2(V) + \cdots .</math>
 
Since the values are in integral cohomology groups, rather than cohomology with real coefficients, these Chern classes are slightly more refined than those in the Riemannian example.
 
===Classical axiomatic definition===
The Chern classes satisfy the following four axioms:
 
'''Axiom 1.''' <math>c_0(V) = 1</math> for all ''V''.
 
'''Axiom 2.''' Functoriality: If <math>f : Y \to X</math> is [[continuous function (topology)|continuous]] and ''f*V'' is the [[pullback bundle|vector bundle pullback]] of ''V'', then <math>c_k(f^* V) = f^* c_k(V)</math>.
 
'''Axiom 3.''' [[Hassler Whitney|Whitney]] sum formula: If <math>W \to X</math> is another complex vector bundle, then the Chern classes of the [[direct sum of vector bundles|direct sum]] <math>V \oplus W</math> are given by
 
:<math>c(V \oplus W) = c(V) \smile c(W);</math>
 
that is,
 
:<math>c_k(V \oplus W) = \sum_{i = 0}^k c_i(V) \smile c_{k - i}(W).</math>
 
'''Axiom 4.''' Normalization: The total Chern class of the [[tautological line bundle]] over '''CP'''<sup>''k''</sup> is 1−''H'', where ''H'' is [[Poincaré duality|Poincaré-dual]] to the [[hyperplane]] <math>\mathbf{CP}^{k - 1} \subseteq \mathbf{CP}^k</math>.
 
===Alexander Grothendieck axiomatic approach===
 
Alternatively, {{harvs|txt|authorlink=Alexander Grothendieck|first=Alexander|last=Grothendieck|year=1958}} replaced these with a slightly smaller set of axioms:
 
* Functoriality:  (Same as above)
 
* Additivity: If <math>\ 0\to E'\to E\to E''\to 0</math> is an [[exact sequence]] of vector bundles, then <math>c(E)=c(E')\smile c(E'')</math>.
 
* Normalization:  If ''E'' is a [[line bundle]], then <math>c(E)=1+e(E_{\mathbf R})</math> where <math>e(E_{\mathbf R})</math> is the [[Euler class]] of the underlying real vector bundle.
 
He shows using the [[Leray-Hirsch theorem]] that the total Chern class of an arbitrary finite rank complex vector bundle can be defined in terms of the first Chern class of a tautologically-defined line bundle.
 
Namely, introducing the projectivization '''P'''(''E'') of the rank ''n'' complex vector bundle ''E'' → ''B'' as the fiber bundle on ''B'' whose fiber at any point <math>b\in B</math> is the projective space of the fiber ''E<sub>b</sub>''. The total space of this bundle '''P'''(''E'') is equipped with its tautological complex line bundle, that we denote τ, and the first Chern class
 
:<math>c_1(\tau)=: -a</math>
 
restricts on each fiber '''P'''(''E<sub>b</sub>'') to minus the (Poincaré-dual) class of the hyperplane, that spans the cohomology of the fiber, in view of the cohomology of [[complex projective space]]s.  
 
The classes
 
:<math>1, a, a^2, \ldots , a^{n-1}\in H^*(\mathbf{P}(E))</math>  
 
therefore form a family of ambient cohomology classes restricting to a basis of the cohomology of the fiber. The [[Leray-Hirsch theorem]] then states that
any class in ''H*''('''P'''(''E'')) can be written uniquely as a linear combination of the 1, ''a'', ''a''<sup>2</sup>, ..., ''a''<sup>''n''−1</sup> with classes on the basis as coefficients.  
 
In particular, one may define the Chern classes of ''E'' in the sense of Grothendieck, denoted <math>c_1(E), \ldots c_{n}(E)</math> by expanding this way the class <math>-a^n</math>, with the relation:
 
:<math> - a^n = c_1(E). a^{n-1}+ \ldots c_{n-1}(E) .a + c_{n}(E) .</math>
 
One then may check that this alternative definition coincides with whatever other definition one may favor, or use the previous axiomatic characterization.
 
===The top Chern class===
In fact, these properties uniquely characterize the Chern classes. They imply, among other things:
 
* If ''n'' is the complex rank of ''V'', then <math>c_k(V) = 0</math> for all ''k'' > ''n''. Thus the total Chern class terminates.
 
* The top Chern class of ''V'' (meaning <math>c_n(V)</math>, where ''n'' is the rank of ''V'') is always equal to the [[Euler class]] of the underlying real vector bundle.
 
==Proximate notions==
 
===The Chern character===
Chern classes can be used to construct a homomorphism of rings from the [[topological K-theory]] of a space to (the completion of) its rational cohomology. For a line bundle ''L'', the Chern character ch is defined by
 
:<math>\operatorname{ch}(L) = \exp(c_{1}(L)) := \sum_{m=0}^\infty \frac{c_1(L)^m}{m!}.</math>
 
More generally, if <math>V = L_1 \oplus ... \oplus L_n</math> is a direct sum of line bundles, with first Chern classes <math>x_i = c_1(L_i),</math> the Chern character is defined additively
 
:<math> \operatorname{ch}(V)  = e^{x_1} + \dots + e^{x_n} :=\sum_{m=0}^\infty \frac{1}{m!}(x_1^m + ... + x_n^m). </math>
 
Observe that when ''V'' is a sum of line bundles, the Chern classes of ''V'' can be expressed as [[elementary symmetric polynomials]] in the <math>x_i</math>, <math>c_i(V) = e_i(x_1,...,x_n).</math>
In particular, on the one hand
:<math>c(V) := \sum_{i=0}^n c_i(V),</math>
while on the other hand
:<math>c(V) = c(L_1 \oplus \dots \oplus L_n) = \prod_{i=1}^n c(L_i) = \prod_{i=1}^n (1+x_i) = \sum_{i=0}^n e_i(x_1,\dots,x_n). </math>
 
Consequently, [[Newton's_identities#Expressing_power_sums_in_terms_of_elementary_symmetric_polynomials|Newton's identities]] may be used to reexpress the power sums in ''ch(V)'' above solely in terms of the Chern classes of ''V'', giving
 
:<math> \operatorname{ch}(V) = \operatorname{dim}(V) + c_1(V) + \frac{1}{2}(c_1(V)^2 - 2c_2(V)) + \frac{1}{6} (c_1(V)^3 - 3c_1(V)c_2(V) + 3c_3(V)) + ..., </math>
 
This last expression, justified by invoking the [[splitting principle]], is taken as the definition ''ch(V)'' for arbitrary vector bundles ''V''.
 
If a connection is used to define the Chern classes, then the explicit form of the Chern character is
: <math>\hbox{ch}(V)=\hbox{tr}\left(\exp\left(\frac{i\Omega}{2\pi}\right)\right)</math>
where Ω is the curvature of the connection.
 
The Chern character is useful in part because it facilitates the computation of the Chern class of a tensor product.  Specifically, it obeys the following identities:
:<math>\hbox{ch}(V\oplus W)=\hbox{ch}(V)+\hbox{ch}(W)</math>
:<math>\hbox{ch}(V\otimes W)=\hbox{ch}(V)\hbox{ch}(W).</math>
 
As stated above, using the Grothendieck additivity axiom for Chern classes, the first of these identities can be generalized to state that ''ch'' is a [[homomorphism]] of [[abelian groups]] from the [[K-theory]] ''K''(''X'') into the rational cohomology of ''X''.  The second identity establishes the fact that this homomorphism also respects products in ''K''(''X''), and so ''ch'' is a homomorphism of rings.
 
The Chern character is used in the [[Hirzebruch-Riemann-Roch theorem]].
 
===Chern numbers===
 
If we work on an [[orientable manifold|oriented manifold]] of dimension ''2n'', then any product of Chern classes of total degree ''2n'' can be paired with the [[orientation homology class]] (or "integrated over the manifold") to give an integer, a '''Chern number''' of the vector bundle. For example, if the manifold has dimension 6, there are three linearly independent Chern numbers, given by ''c''<sub>1</sub><sup>3</sup>, ''c''<sub>1</sub>''c''<sub>2</sub>, and ''c''<sub>3</sub>. In general, if the manifold has dimension 2''n'', the number of possible independent Chern numbers is the number of [[integer partition|partition]]s of ''n''.
 
The Chern numbers of the tangent bundle of a complex (or almost complex) manifold are called the Chern numbers of the manifold, and are important invariants.
 
===The Chern class in generalized cohomology theories===
 
There is a generalization of the theory of Chern classes, where ordinary cohomology is replaced with a [[generalized cohomology theory]].  The theories for which such generalization is possible are called ''[[Complex_cobordism#Formal group laws|complex orientable]]''. The formal properties of the Chern classes remain the same, with one crucial difference: the rule which computes the first Chern class of a tensor product of line bundles in terms of first Chern classes of the factors is not (ordinary) addition, but rather a [[formal group law]].
 
=== Chern classes of manifolds with structure ===
 
The theory of Chern classes gives rise to [[cobordism]] invariants for [[almost complex manifold]]s.
 
If ''M'' is an almost complex manifold, then its [[tangent bundle]] is a complex vector bundle. The '''Chern classes''' of ''M'' are thus defined to be the Chern classes of its tangent bundle.  If ''M'' is also [[Compact space|compact]] and of dimension 2''d'', then each [[monomial]] of total degree 2''d'' in the Chern classes can be paired with the [[fundamental class]] of ''M'', giving an integer, a '''Chern number''' of ''M''. If ''M''′ is another almost complex manifold of the same dimension, then it is cobordant to ''M'' if and only if the Chern numbers of ''M''′ coincide with those of ''M''.
 
The theory also extends to real [[Symplectic geometry|symplectic]] vector bundles, by the intermediation of compatible almost complex structures. In particular, [[symplectic manifold]]s have a well-defined Chern class.
 
=== Chern classes on arithmetic schemes and Diophantine equations ===
 
(See [[Arakelov geometry]])
 
== See also ==
* [[Pontryagin class]]
* [[Stiefel-Whitney class]]
* [[Euler class]]
* [[Segre class]]
 
==References==
* {{Citation | last1=Chern | first1=S. S. | title=Characteristic classes of Hermitian Manifolds | year=1946 | journal=[[Annals of Mathematics|Annals of Mathematics. Second Series]] | issn=0003-486X | volume=47 | issue=1 | pages=85–121 | doi=10.2307/1969037 | publisher=The Annals of Mathematics, Vol. 47, No. 1 |jstor=1969037}}
* {{Citation | last1=Grothendieck | first1=Alexander | author1-link=Alexander Grothendieck | title=La théorie des classes de Chern | mr=0116023 | year=1958 | journal=Bulletin de la Société Mathématique de France | issn=0037-9484 | volume=86 | pages=137–154
|url= http://www.numdam.org/item?id=BSMF_1958__86__137_0}}
* {{Citation | last1=Jost | first1=Jürgen | title=Riemannian Geometry and Geometric Analysis | publisher=[[Springer-Verlag]] | location=Berlin, New York | edition=4th | isbn=978-3-540-25907-7 | year=2005}} (Provides a very short, introductory review of Chern classes).
* {{Citation | last1=Milnor | first1=John Willard | author1-link=John Milnor | last2=Stasheff | first2=James D. |author2-link=Jim Stasheff| title=Characteristic classes | publisher=Princeton University Press; University of Tokyo Press | series=Annals of Mathematics Studies | isbn=978-0-691-08122-9 | year=1974 | volume=76}}
 
==External links==
* [http://www.math.cornell.edu/~hatcher/VBKT/VBpage.html Vector Bundles & K-Theory] - A downloadable book-in-progress by [[Allen Hatcher]]. Contains a chapter about characteristic classes.
*[[Dieter Kotschick]], [http://www.physorg.com/news163858041.html Chern numbers of algebraic varieties]
 
[[Category:Characteristic classes]]

Revision as of 16:36, 22 September 2013

In mathematics, in particular in algebraic topology, differential geometry and algebraic geometry, the Chern classes are characteristic classes associated to complex vector bundles.

Chern classes were introduced by Template:Harvs.

Geometric approach

Basic idea and motivation

Chern classes are characteristic classes. They are topological invariants associated to vector bundles on a smooth manifold. The question of whether two ostensibly different vector bundles are the same can be quite hard to answer. The Chern classes provide a simple test: if the Chern classes of a pair of vector bundles do not agree, then the vector bundles are different. The converse, however, is not true.

In topology, differential geometry, and algebraic geometry, it is often important to count how many linearly independent sections a vector bundle has. The Chern classes offer some information about this through, for instance, the Riemann-Roch theorem and the Atiyah-Singer index theorem.

Chern classes are also feasible to calculate in practice. In differential geometry (and some types of algebraic geometry), the Chern classes can be expressed as polynomials in the coefficients of the curvature form.

Construction of Chern classes

There are various ways of approaching the subject, each of which focuses on a slightly different flavor of Chern class.

The original approach to Chern classes was via algebraic topology: the Chern classes arise via homotopy theory which provides a mapping associated to V to a classifying space (an infinite Grassmannian in this case). Any vector bundle V over a manifold may be realized as the pullback of a universal bundle over the classifying space, and the Chern classes of V can therefore be defined as the pullback of the Chern classes of the universal bundle; these universal Chern classes in turn can be explicitly written down in terms of Schubert cycles.

Chern's approach used differential geometry, via the curvature approach described predominantly in this article. He showed that the earlier definition was in fact equivalent to his.

There is also an approach of Alexander Grothendieck showing that axiomatically one need only define the line bundle case.

Chern classes arise naturally in algebraic geometry. The generalized Chern classes in algebraic geometry can be defined for vector bundles (or more precisely, locally free sheaves) over any nonsingular variety. Algebro-geometric Chern classes do not require the underlying field to have any special properties. In particular, the vector bundles need not necessarily be complex.

Regardless of the particular paradigm, the intuitive meaning of the Chern class concerns 'required zeroes' of a section of a vector bundle: for example the theorem saying one can't comb a hairy ball flat (hairy ball theorem). Although that is strictly speaking a question about a real vector bundle (the "hairs" on a ball are actually copies of the real line), there are generalizations in which the hairs are complex (see the example of the complex hairy ball theorem below), or for 1-dimensional projective spaces over many other fields.

See Chern-Simons for more discussion.

The Chern class of line bundles

28 year-old Painting Investments Worker Truman from Regina, usually spends time with pastimes for instance interior design, property developers in new launch ec Singapore and writing. Last month just traveled to City of the Renaissance.

An important special case occurs when V is a line bundle. Then the only nontrivial Chern class is the first Chern class, which is an element of the second cohomology group of X. As it is the top Chern class, it equals the Euler class of the bundle.

The first Chern class turns out to be a complete invariant with which to classify complex line bundles, topologically speaking. That is, there is a bijection between the isomorphism classes of line bundles over X and the elements of H2(X;Z), which associates to a line bundle its first Chern class. Addition in the second dimensional cohomology group coincides with tensor product of complex line bundles. Potter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park.

In algebraic geometry, this classification of (isomorphism classes of) complex line bundles by the first Chern class is a crude approximation to the classification of (isomorphism classes of) holomorphic line bundles by linear equivalence classes of divisors.

For complex vector bundles of dimension greater than one, the Chern classes are not a complete invariant.

The Chern class in Chern-Weil theory

The Chern class of a Hermitian vector bundle on a smooth manifold

Given a complex hermitian vector bundle V of complex rank n over a smooth manifold M, a representative of each Chern class (also called a Chern form) ck(V) of V are given as the coefficients of the characteristic polynomial of the curvature form Ω of V.

The determinant is over the ring of n × n matrices whose entries are polynomials in t with coefficients in the commutative algebra of even complex differential forms on M. The curvature form Ω of V is defined as

with ω the connection form and d the exterior derivative, or via the same expression in which ω is a gauge form for the gauge group of V. The scalar t is used here only as an indeterminate to generate the sum from the determinant, and I denotes the n × n identity matrix.

To say that the expression given is a representative of the Chern class indicates that 'class' here means up to addition of an exact differential form. That is, Chern classes are cohomology classes in the sense of de Rham cohomology. It can be shown that the cohomology class of the Chern forms do not depend on the choice of connection in V.

Using the matrix identity tr(ln(X))=ln(det(X)) and the Maclaurin series for ln(X+I), this expression for the Chern form expands as

Example: the complex tangent bundle of the Riemann sphere

Let CP1 be the Riemann sphere: 1-dimensional complex projective space. Suppose that z is a holomorphic local coordinate for the Riemann sphere. Let V = TCP1 be the bundle of complex tangent vectors having the form a∂/∂z at each point, where a is a complex number. We prove the complex version of the hairy ball theorem: V has no section which is everywhere nonzero.

For this, we need the following fact: the first Chern class of a trivial bundle is zero, i.e.,

This is evinced by the fact that a trivial bundle always admits a flat connection.

So, we shall show that

Consider the Kähler metric

One readily shows that the curvature 2-form is given by

Furthermore, by the definition of the first Chern class

We must show that this cohomology class is non-zero. It suffices to compute its integral over the Riemann sphere:

after switching to polar coordinates. By Stokes' theorem, an exact form would integrate to 0, so the cohomology class is nonzero.

This proves that TCP1 is not a trivial vector bundle.

Properties of Chern classes

Given a complex vector bundle V over a topological space X, the Chern classes of V are a sequence of elements of the cohomology of X. The k-th Chern class of V, which is usually denoted ck(V), is an element of

H2k(X;Z),

the cohomology of X with integer coefficients. One can also define the total Chern class

Since the values are in integral cohomology groups, rather than cohomology with real coefficients, these Chern classes are slightly more refined than those in the Riemannian example.

Classical axiomatic definition

The Chern classes satisfy the following four axioms:

Axiom 1. for all V.

Axiom 2. Functoriality: If is continuous and f*V is the vector bundle pullback of V, then .

Axiom 3. Whitney sum formula: If is another complex vector bundle, then the Chern classes of the direct sum are given by

that is,

Axiom 4. Normalization: The total Chern class of the tautological line bundle over CPk is 1−H, where H is Poincaré-dual to the hyperplane .

Alexander Grothendieck axiomatic approach

Alternatively, Template:Harvs replaced these with a slightly smaller set of axioms:

  • Functoriality: (Same as above)

He shows using the Leray-Hirsch theorem that the total Chern class of an arbitrary finite rank complex vector bundle can be defined in terms of the first Chern class of a tautologically-defined line bundle.

Namely, introducing the projectivization P(E) of the rank n complex vector bundle EB as the fiber bundle on B whose fiber at any point is the projective space of the fiber Eb. The total space of this bundle P(E) is equipped with its tautological complex line bundle, that we denote τ, and the first Chern class

restricts on each fiber P(Eb) to minus the (Poincaré-dual) class of the hyperplane, that spans the cohomology of the fiber, in view of the cohomology of complex projective spaces.

The classes

therefore form a family of ambient cohomology classes restricting to a basis of the cohomology of the fiber. The Leray-Hirsch theorem then states that any class in H*(P(E)) can be written uniquely as a linear combination of the 1, a, a2, ..., an−1 with classes on the basis as coefficients.

In particular, one may define the Chern classes of E in the sense of Grothendieck, denoted by expanding this way the class , with the relation:

One then may check that this alternative definition coincides with whatever other definition one may favor, or use the previous axiomatic characterization.

The top Chern class

In fact, these properties uniquely characterize the Chern classes. They imply, among other things:

Proximate notions

The Chern character

Chern classes can be used to construct a homomorphism of rings from the topological K-theory of a space to (the completion of) its rational cohomology. For a line bundle L, the Chern character ch is defined by

More generally, if is a direct sum of line bundles, with first Chern classes the Chern character is defined additively

Observe that when V is a sum of line bundles, the Chern classes of V can be expressed as elementary symmetric polynomials in the , In particular, on the one hand

while on the other hand

Consequently, Newton's identities may be used to reexpress the power sums in ch(V) above solely in terms of the Chern classes of V, giving

This last expression, justified by invoking the splitting principle, is taken as the definition ch(V) for arbitrary vector bundles V.

If a connection is used to define the Chern classes, then the explicit form of the Chern character is

where Ω is the curvature of the connection.

The Chern character is useful in part because it facilitates the computation of the Chern class of a tensor product. Specifically, it obeys the following identities:

As stated above, using the Grothendieck additivity axiom for Chern classes, the first of these identities can be generalized to state that ch is a homomorphism of abelian groups from the K-theory K(X) into the rational cohomology of X. The second identity establishes the fact that this homomorphism also respects products in K(X), and so ch is a homomorphism of rings.

The Chern character is used in the Hirzebruch-Riemann-Roch theorem.

Chern numbers

If we work on an oriented manifold of dimension 2n, then any product of Chern classes of total degree 2n can be paired with the orientation homology class (or "integrated over the manifold") to give an integer, a Chern number of the vector bundle. For example, if the manifold has dimension 6, there are three linearly independent Chern numbers, given by c13, c1c2, and c3. In general, if the manifold has dimension 2n, the number of possible independent Chern numbers is the number of partitions of n.

The Chern numbers of the tangent bundle of a complex (or almost complex) manifold are called the Chern numbers of the manifold, and are important invariants.

The Chern class in generalized cohomology theories

There is a generalization of the theory of Chern classes, where ordinary cohomology is replaced with a generalized cohomology theory. The theories for which such generalization is possible are called complex orientable. The formal properties of the Chern classes remain the same, with one crucial difference: the rule which computes the first Chern class of a tensor product of line bundles in terms of first Chern classes of the factors is not (ordinary) addition, but rather a formal group law.

Chern classes of manifolds with structure

The theory of Chern classes gives rise to cobordism invariants for almost complex manifolds.

If M is an almost complex manifold, then its tangent bundle is a complex vector bundle. The Chern classes of M are thus defined to be the Chern classes of its tangent bundle. If M is also compact and of dimension 2d, then each monomial of total degree 2d in the Chern classes can be paired with the fundamental class of M, giving an integer, a Chern number of M. If M′ is another almost complex manifold of the same dimension, then it is cobordant to M if and only if the Chern numbers of M′ coincide with those of M.

The theory also extends to real symplectic vector bundles, by the intermediation of compatible almost complex structures. In particular, symplectic manifolds have a well-defined Chern class.

Chern classes on arithmetic schemes and Diophantine equations

(See Arakelov geometry)

See also

References

  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 (Provides a very short, introductory review of Chern classes).
  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010

External links