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In [[mathematics]] algebraic '''L-theory''' is the [[K-theory]] of [[quadratic form]]s; the term was coined by [[C. T. C. Wall]],
with ''L'' being used as the letter after ''K''. Algebraic ''L''-theory, also known as 'hermitian ''K''-theory',
is important in [[surgery theory]].


==Definition==
One can define ''L''-groups for any [[ring with involution]] ''R'': the quadratic ''L''-groups <math>L_*(R)</math> (Wall) and the symmetric ''L''-groups <math>L^*(R)</math> (Mishchenko, Ranicki).


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=== Even dimension ===
The even dimensional ''L''-groups <math>L_{2k}(R)</math> are defined as the [[Witt group]]s of [[ε-quadratic forms]] over the ring ''R'' with <math>\epsilon = (-1)^k</math>. More precisely,
 
<math>L_{2k}(R)</math>
 
is the abelian group of equivalence classes <math>[\psi]</math> of non-degenerate ε-quadratic forms <math>\psi \in Q_\epsilon(F)</math> over R, where the underlying R-modules F are finitely generated free. The equivalence relation is given by stabilization with respect to [[hyperbolic ε-quadratic forms]]:
 
:<math>[\psi] = [\psi'] \Longleftrightarrow n, n' \in {\mathbb N}_0: \psi \oplus H_{(-1)^k}(R)^n \cong \psi' \oplus H_{(-1)^k}(R)^{n'}</math>.
 
The addition in <math>L_{2k}(R)</math> is defined by
 
:<math>[\psi_1] + [\psi_2] := [\psi_1 \oplus \psi_2].</math>
 
The zero element is represented by <math>H_{(-1)^k}(R)^n</math> for any <math>n \in {\mathbb N}_0</math>. The inverse of <math>[\psi]</math> is <math>[-\psi]</math>.
 
=== Odd dimension ===
Defining odd dimensional ''L''-groups is more complicated; further details and the definition of the odd dimensional ''L''-groups can be found in the references mentioned below.
 
==Examples and applications==
The ''L''-groups of a group <math>\pi</math> are the ''L''-groups <math>L_*(\mathbf{Z}[\pi])</math> of the [[group ring]] <math>\mathbf{Z}[\pi]</math>. In the applications to topology  <math>\pi</math> is the [[fundamental group]]
<math>\pi_1 (X)</math> of a space <math>X</math>. The quadratic ''L''-groups <math>L_*(\mathbf{Z}[\pi])</math>
play a central role in the surgery classification of the homotopy types of <math>n</math>-dimensional [[manifolds]] of dimension <math>n > 4</math>, and in the formulation of the [[Novikov conjecture]].
 
The distinction between symmetric ''L''-groups and quadratic ''L''-groups, indicated by upper and lower indices, reflects the usage in group homology and cohomology. The [[group cohomology]] <math>H^*</math> of the cyclic group <math>\mathbf{Z}_2</math> deals with the fixed points of a <math>\mathbf{Z}_2</math>-action, while the [[group homology]] <math>H_*</math> deals with the orbits of a <math>\mathbf{Z}_2</math>-action; compare <math>X^G</math> (fixed points) and <math>X_G = X/G</math> (orbits, quotient) for upper/lower index notation.
 
The quadratic ''L''-groups: <math>L_n(R)</math> and the symmetric ''L''-groups: <math>L^n(R)</math> are related by
a symmetrization map <math>L_n(R) \to L^n(R)</math> which is an isomorphism modulo 2-torsion, and which corresponds to the [[polarization identities]].
 
The quadratic ''L''-groups are 4-fold periodic. Symmetric ''L''-groups are not 4-periodic in general (see Ranicki, page 12), though they are for the integers.
 
In view of the applications to the [[classification of manifolds]] there are extensive calculations of
the quadratic <math>L</math>-groups <math>L_*(\mathbf{Z}[\pi])</math>. For finite <math>\pi</math>
algebraic methods are used, and mostly geometric methods (e.g. controlled topology) are used for infinite <math>\pi</math>.  
 
More generally, one can define ''L''-groups for any [[additive category]] with a ''chain duality'', as in Ranicki (section 1).
 
=== Integers ===
The '''simply connected ''L''-groups''' are also the ''L''-groups of the integers, as
<math>L(e) := L(\mathbf{Z}[e]) = L(\mathbf{Z})</math> for both <math>L</math> = <math>L^*</math> or <math>L_*.</math> For quadratic ''L''-groups, these are the surgery obstructions to [[simply connected]] surgery.
 
The quadratic ''L''-groups of the integers are:
:<math>\begin{align}
L_{4k}(\mathbf{Z}) &= \mathbf{Z}  && \text{signature}/8\\
L_{4k+1}(\mathbf{Z}) &= 0\\
L_{4k+2}(\mathbf{Z}) &= \mathbf{Z}/2 && \text{Arf invariant}\\
L_{4k+3}(\mathbf{Z}) &= 0.
\end{align}</math>
In [[doubly even]] dimension (4''k''), the quadratic ''L''-groups detect the [[signature (topology)|signature]]; in [[singly even]] dimension (4''k''+2), the ''L''-groups detect the [[Arf invariant]] (topologically the [[Kervaire invariant]]).
 
The symmetric ''L''-groups of the integers are:
:<math>\begin{align}
L^{4k}(\mathbf{Z}) &= \mathbf{Z} && \text{signature}\\
L^{4k+1}(\mathbf{Z}) &= \mathbf{Z}/2 && \text{de Rham invariant}\\
L^{4k+2}(\mathbf{Z}) &= 0\\
L^{4k+3}(\mathbf{Z}) &= 0.
\end{align}</math>
In doubly even dimension (4''k''), the symmetric ''L''-groups, as with the quadratic ''L''-groups, detect the signature; in dimension (4''k''+1), the ''L''-groups detect the [[de Rham invariant]].
 
==References==
 
*{{Citation | last1=Lück | first1=Wolfgang | title=Topology of high-dimensional manifolds, No. 1, 2 (Trieste, 2001) | url=http://www.math.uni-muenster.de/u/lueck/publ/lueck/ictp.pdf | publisher=Abdus Salam Int. Cent. Theoret. Phys., Trieste | series=ICTP Lect. Notes | mr=1937016 | year=2002 | volume=9 | chapter=A basic introduction to surgery theory | pages=1–224}}
*{{Citation | last1=Ranicki | first1=A. A. | title=Algebraic L-theory and topological manifolds | url=http://www.maths.ed.ac.uk/~aar/books/topman.pdf | publisher=[[Cambridge University Press]] | series=Cambridge Tracts in Mathematics | isbn=978-0-521-42024-2 | mr=1211640 | year=1992 | volume=102}}
*{{Citation | last1=Wall | first1=C. T. C. | editor1-last=Ranicki | editor1-first=Andrew | title=Surgery on compact manifolds | origyear=1970 | url=http://www.maths.ed.ac.uk/~aar/books/scm.pdf | publisher=[[American Mathematical Society]] | location=Providence, R.I. | edition=2nd | series=Mathematical Surveys and Monographs | isbn=978-0-8218-0942-6 | mr=1687388 | year=1999 | volume=69}}
 
[[Category:Geometric topology]]
[[Category:Algebraic topology]]
[[Category:Quadratic forms]]
[[Category:Surgery theory]]

Revision as of 12:02, 10 January 2014

In mathematics algebraic L-theory is the K-theory of quadratic forms; the term was coined by C. T. C. Wall, with L being used as the letter after K. Algebraic L-theory, also known as 'hermitian K-theory', is important in surgery theory.

Definition

One can define L-groups for any ring with involution R: the quadratic L-groups L*(R) (Wall) and the symmetric L-groups L*(R) (Mishchenko, Ranicki).

Even dimension

The even dimensional L-groups L2k(R) are defined as the Witt groups of ε-quadratic forms over the ring R with ϵ=(1)k. More precisely,

L2k(R)

is the abelian group of equivalence classes [ψ] of non-degenerate ε-quadratic forms ψQϵ(F) over R, where the underlying R-modules F are finitely generated free. The equivalence relation is given by stabilization with respect to hyperbolic ε-quadratic forms:

[ψ]=[ψ]n,n0:ψH(1)k(R)nψH(1)k(R)n.

The addition in L2k(R) is defined by

[ψ1]+[ψ2]:=[ψ1ψ2].

The zero element is represented by H(1)k(R)n for any n0. The inverse of [ψ] is [ψ].

Odd dimension

Defining odd dimensional L-groups is more complicated; further details and the definition of the odd dimensional L-groups can be found in the references mentioned below.

Examples and applications

The L-groups of a group π are the L-groups L*(Z[π]) of the group ring Z[π]. In the applications to topology π is the fundamental group π1(X) of a space X. The quadratic L-groups L*(Z[π]) play a central role in the surgery classification of the homotopy types of n-dimensional manifolds of dimension n>4, and in the formulation of the Novikov conjecture.

The distinction between symmetric L-groups and quadratic L-groups, indicated by upper and lower indices, reflects the usage in group homology and cohomology. The group cohomology H* of the cyclic group Z2 deals with the fixed points of a Z2-action, while the group homology H* deals with the orbits of a Z2-action; compare XG (fixed points) and XG=X/G (orbits, quotient) for upper/lower index notation.

The quadratic L-groups: Ln(R) and the symmetric L-groups: Ln(R) are related by a symmetrization map Ln(R)Ln(R) which is an isomorphism modulo 2-torsion, and which corresponds to the polarization identities.

The quadratic L-groups are 4-fold periodic. Symmetric L-groups are not 4-periodic in general (see Ranicki, page 12), though they are for the integers.

In view of the applications to the classification of manifolds there are extensive calculations of the quadratic L-groups L*(Z[π]). For finite π algebraic methods are used, and mostly geometric methods (e.g. controlled topology) are used for infinite π.

More generally, one can define L-groups for any additive category with a chain duality, as in Ranicki (section 1).

Integers

The simply connected L-groups are also the L-groups of the integers, as L(e):=L(Z[e])=L(Z) for both L = L* or L*. For quadratic L-groups, these are the surgery obstructions to simply connected surgery.

The quadratic L-groups of the integers are:

L4k(Z)=Zsignature/8L4k+1(Z)=0L4k+2(Z)=Z/2Arf invariantL4k+3(Z)=0.

In doubly even dimension (4k), the quadratic L-groups detect the signature; in singly even dimension (4k+2), the L-groups detect the Arf invariant (topologically the Kervaire invariant).

The symmetric L-groups of the integers are:

L4k(Z)=ZsignatureL4k+1(Z)=Z/2de Rham invariantL4k+2(Z)=0L4k+3(Z)=0.

In doubly even dimension (4k), the symmetric L-groups, as with the quadratic L-groups, detect the signature; in dimension (4k+1), the L-groups detect the de Rham invariant.

References

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    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.

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