Rational quadratic covariance function: Difference between revisions

Latest revision as of 08:16, 8 January 2013

In mathematics, a symmetric tensor is a tensor that is invariant under a permutation of its vector arguments:

T (v_{1}, v_{2}, \dots, v_{r}) = T (v_{σ 1}, v_{σ 2}, \dots, v_{σ r})

for every permutation σ of the symbols {1,2,...,r}. Alternatively, an r^th order symmetric tensor represented in coordinates as a quantity with r indices satisfies

T_{i_{1} i_{2} \dots i_{r}} = T_{i_{σ 1} i_{σ 2} \dots i_{σ r}} .

The space of symmetric tensors of rank r on a finite dimensional vector space is naturally isomorphic to the dual of the space of homogeneous polynomials of degree r on V. Over fields of characteristic zero, the graded vector space of all symmetric tensors can be naturally identified with the symmetric algebra on V. A related concept is that of the antisymmetric tensor or alternating form. Symmetric tensors occur widely in engineering, physics and mathematics.

Definition

Let V be a vector space and

T \in V^{\otimes k}

a tensor of order k. Then T is a symmetric tensor if

τ_{σ} T = T

for the braiding maps associated to every permutation σ on the symbols {1,2,...,k} (or equivalently for every transposition on these symbols).

Given a basis {eⁱ} of V, any symmetric tensor T of rank k can be written as

T = \sum_{i_{1}, \dots, i_{k} = 1}^{N} T_{i_{1} i_{2} \dots i_{k}} e^{i_{1}} \otimes e^{i_{2}} \otimes \dots \otimes e^{i_{k}}

for some unique list of coefficients $T_{i_{1} i_{2} \dots i_{k}}$ (the components of the tensor in the basis) that are symmetric on the indices. That is to say

T_{i_{σ 1} i_{σ 2} \dots i_{σ k}} = T_{i_{1} i_{2} \dots i_{k}}

for every permutation σ.

The space of all symmetric tensors of order k defined on V is often denoted by S^k(V) or Sym^k(V). It is itself a vector space, and if V has dimension N then the dimension of Sym^k(V) is the binomial coefficient

\dim {Sym}^{k} (V) = (\binom{N + k - 1}{k}) .

We then construct Sym(V) as the direct sum of Sym^k(V) for k = 0,1,2,…

Sym (V) = ⨁_{k = 0}^{\infty} {Sym}^{k} (V) .

Examples

Many material properties and fields used in physics and engineering can be represented as symmetric tensor fields; for example: stress, strain, and anisotropic conductivity. Also, in diffusion MRI one often uses symmetric tensors to describe diffusion in the brain or other parts of the body.

Ellipsoids are examples of algebraic varieties; and so, for general rank, symmetric tensors, in the guise of homogeneous polynomials, are used to define projective varieties, and are often studied as such.

Symmetric part of a tensor

Suppose $V$ is a vector space over a field of characteristic 0. If T∈V^⊗k is a tensor of order $k$ , then the symmetric part of $T$ is the symmetric tensor defined by

Sym T = \frac{1}{k!} \sum_{σ \in S_{k}} τ_{σ} T,

the summation extending over the symmetric group on k symbols. In terms of a basis, and employing the Einstein summation convention, if

T = T_{i_{1} i_{2} \dots i_{k}} e^{i_{1}} \otimes e^{i_{2}} \otimes \dots \otimes e^{i_{k}},

then

Sym T = \frac{1}{k!} \sum_{σ \in S_{k}} T_{i_{σ 1} i_{σ 2} \dots i_{σ k}} e^{i_{1}} \otimes e^{i_{2}} \otimes \dots \otimes e^{i_{k}} .

The components of the tensor appearing on the right are often denoted by

T_{(i_{1} i_{2} \dots i_{k})} = \frac{1}{k!} \sum_{σ \in S_{k}} T_{i_{σ 1} i_{σ 2} \dots i_{σ k}}

with parentheses around the indices which have been symmetrized. [Square brackets are used to indicate anti-symmetrization.]

Symmetric product

If T is a simple tensor, given as a pure tensor product

T = v_{1} \otimes v_{2} \otimes \dots \otimes v_{r}

then the symmetric part of T is the symmetric product of the factors:

v_{1} ⊙ v_{2} ⊙ \dots ⊙ v_{r} : = \frac{1}{r!} \sum_{σ \in S_{r}} v_{i_{σ 1}} \otimes v_{i_{σ 2}} \otimes \dots \otimes v_{i_{σ r}} .

In general we can turn Sym(V) into an algebra by defining the commutative and associative product ' $⊙$ '.^[1] Given two tensors T₁∈Sym^k₁(V) and T₂∈Sym^k₂(V), we use the symmetrization operator to define:

T_{1} ⊙ T_{2} = Sym (T_{1} \otimes T_{2}) (\in {Sym}^{k_{1} + k_{2}} (V)) .

It can be verified (as is done by Kostrikin and Manin^[1]) that the resulting product is in fact commutative and associative. In some cases the operator is not written at all: T₁T₂ = T₁ $⊙$ T₂.

In some cases an exponential notation is used:

v^{⊙ k} = \underset{k times}{\underset{⏟}{v ⊙ v ⊙ \dots ⊙ v}} = \underset{k times}{\underset{⏟}{v \otimes v \otimes \dots \otimes v}} = v^{\otimes k} .

Where v is a vector. Again, in some cases the ' $⊙$ ' is left out:

v^{k} = \underset{k times}{\underset{⏟}{v v \dots v}} = \underset{k times}{\underset{⏟}{v ⊙ v ⊙ \dots ⊙ v}} .

Decomposition

In analogy with the theory of symmetric matrices, a (real) symmetric tensor of order 2 can be "diagonalized". More precisely, for any tensor T ∈ Sym²(V), there are an integer r, non-zero unit vectors v₁,...,v_r ∈ V and weights λ₁,...,λ_r such that

T = \sum_{i = 1}^{r} λ_{i} v_{i} \otimes v_{i} .

The minimum number r for which such a decomposition is possible is the (symmetric) rank of T. The vectors appearing in this minimal expression are the principal axes of the tensor, and generally have an important physical meaning. For example, the principal axes of the inertia tensor define the Poinsot's ellipsoid representing the moment of inertia. Also see Sylvester's law of inertia.

For symmetric tensors of arbitrary order k, decompositions

T = \sum_{i = 1}^{r} λ_{i} v_{i}^{\otimes k}

are also possible. The minimum number r for which such a decomposition is possible is the symmetric rank of T.^[2] For second order tensors this corresponds to the rank of the matrix representing the tensor in any basis, and it is well-known that the maximum rank is equal to the dimension of the underlying vector space. However, for higher orders this need not hold: the rank can be higher than the number of dimensions in the underlying vector space. The higher-order singular value decomposition of a symmetric tensor is a special decomposition of this form ^[2] (often called the canonical decomposition.)

Notes

↑ ^1.0 ^1.1 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
↑ ^2.0 ^2.1 Template:Cite doi

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.
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

Cesar O. Aguilar, The Dimension of Symmetric k-tensors

Template:Tensors

[Kostrikin1997-1] 1.0 ^1.1 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

My blog: http://www.primaboinca.com/view_profile.php?userid=5889534

[Comon2008-2] 2.0 ^2.1 Template:Cite doi

[1]

[2]

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+In [[mathematics]], a '''symmetric tensor''' is a [[tensor]] that is invariant under a [[permutation]] of its vector arguments:
+:<math>T(v_1,v_2,\dots,v_r) = T(v_{\sigma 1},v_{\sigma 2},\dots,v_{\sigma r})</math>
+for every permutation &sigma; of the symbols {1,2,...,''r''}.
+Alternatively, an ''r''<sup>th</sup> order symmetric tensor represented in coordinates as a quantity with ''r'' indices satisfies
+:<math>T_{i_1i_2\dots i_r} = T_{i_{\sigma 1}i_{\sigma 2}\dots i_{\sigma r}}.</math>
+The space of symmetric tensors of rank ''r'' on a finite dimensional [[vector space]] is [[natural isomorphism|naturally isomorphic]] to the dual of the space of [[homogeneous polynomial]]s of degree ''r'' on ''V''.  Over [[field (mathematics)|fields]] of [[characteristic zero]], the [[graded vector space]] of all symmetric tensors can be naturally identified with the [[symmetric algebra]] on ''V''. A related concept is that of the [[antisymmetric tensor]] or [[alternating form]]. Symmetric tensors occur widely in [[engineering]], [[physics]] and [[mathematics]].
+==Definition==
+Let ''V'' be a vector space and
+:<math>T\in V^{\otimes k}</math>
+a tensor of order ''k''.  Then ''T'' is a symmetric tensor if
+:<math>\tau_\sigma T = T\,</math>
+for the [[Tensor product#Tensor powers and braiding|braiding maps]] associated to every permutation &sigma; on the symbols {1,2,...,''k''} (or equivalently for every [[Transposition (mathematics)|transposition]] on these symbols).
+Given a [[basis (linear algebra)|basis]] {''e''<sup>''i''</sup>} of ''V'', any symmetric tensor ''T'' of rank ''k'' can be written as
+:<math>T = \sum_{i_1,\dots,i_k=1}^N T_{i_1i_2\dots i_k} e^{i_1} \otimes e^{i_2}\otimes\cdots \otimes e^{i_k}</math>
+for some unique list of coefficients <math>T_{i_1i_2\dots i_k}</math> (the ''components'' of the tensor in the basis) that are symmetric on the indices.  That is to say
+:<math>T_{i_{\sigma 1}i_{\sigma 2}\dots i_{\sigma k}} = T_{i_1i_2\dots i_k}</math>
+for every [[permutation]] &sigma;.
+The space of all symmetric tensors of order ''k'' defined on ''V'' is often denoted by ''S''<sup>''k''</sup>(''V'') or Sym<sup>''k''</sup>(''V'').  It is itself a vector space, and if ''V'' has dimension ''N'' then  the dimension of Sym<sup>''k''</sup>(''V'') is the [[binomial coefficient]]
+:<math>\dim\, \operatorname{Sym}^k(V) = {N + k - 1 \choose k}.</math>
+We then construct Sym(''V'') as the [[direct sum of vector spaces|direct sum]] of Sym<sup>''k''</sup>(''V'') for ''k'' = 0,1,2,…
+:<math>\operatorname{Sym}(V)= \bigoplus_{k=0}^\infty \operatorname{Sym}^k(V).</math>
+==Examples==
+Many [[material properties]] and [[field (physics)|fields]] used in physics and engineering can be represented as symmetric tensor fields; for example: [[stress (physics)|stress]], [[strain tensor|strain]], and [[anisotropic]] [[Electrical resistivity and conductivity|conductivity]]. Also, in [[diffusion MRI]] one often uses symmetric tensors to describe diffusion in the brain or other parts of the body.
+Ellipsoids are examples of [[algebraic varieties]]; and so, for general rank,  symmetric tensors, in the guise of [[homogeneous polynomial]]s, are used to define [[projective varieties]], and are often studied as such.
+==Symmetric part of a tensor==
+Suppose <math>V</math> is a vector space over a field of [[Characteristic (algebra)|characteristic]] 0.  If ''T''&isin;''V''<sup>&otimes;''k''</sup> is a tensor of order <math>k</math>, then the symmetric part of <math>T</math> is the symmetric tensor defined by
+:<math>\operatorname{Sym}\, T = \frac{1}{k!}\sum_{\sigma\in\mathfrak{S}_k} \tau_\sigma T,</math>
+the summation extending over the [[symmetric group]] on ''k'' symbols.  In terms of a basis, and employing the [[Einstein summation convention]], if
+:<math>T = T_{i_1i_2\dots i_k}e^{i_1}\otimes e^{i_2}\otimes\cdots \otimes e^{i_k},</math>
+then
+:<math>\operatorname{Sym}\, T = \frac{1}{k!}\sum_{\sigma\in \mathfrak{S}_k} T_{i_{\sigma 1}i_{\sigma 2}\dots i_{\sigma k}} e^{i_1}\otimes e^{i_2}\otimes\cdots \otimes e^{i_k}.</math>
+The components of the tensor appearing on the right are often denoted by
+:<math>T_{(i_1i_2\dots i_k)} = \frac{1}{k!}\sum_{\sigma\in \mathfrak{S}_k} T_{i_{\sigma 1}i_{\sigma 2}\dots i_{\sigma k}}</math>
+with parentheses around the indices which have been symmetrized. [Square brackets are used to indicate anti-symmetrization.]
+==Symmetric product==
+If ''T'' is a simple tensor, given as a pure tensor product
+:<math>T=v_1\otimes v_2\otimes\cdots \otimes v_r</math>
+then the symmetric part of ''T'' is the symmetric product of the factors:
+:<math>v_1\odot v_2\odot\cdots\odot v_r := \frac{1}{r!}\sum_{\sigma\in\mathfrak{S}_r} v_{i_{\sigma 1}}\otimes v_{i_{\sigma 2}}\otimes\cdots\otimes v_{i_{\sigma r}}.</math>
+In general we can turn Sym(''V'') into an [[algebra]] by defining the commutative and associative product '<math>\odot</math>'.<ref name="Kostrikin1997">{{cite book
+ | last1 = Kostrikin | first1 = Alexei I.
+ | last2 = Manin | first2 = Iurii Ivanovich
+ | authorlink1 = Alexei Kostrikin
+ | authorlink2 = Yuri I. Manin
+ | title = Linear algebra and geometry
+ | publisher = Gordon and Breach
+ | series = Algebra, Logic and Applications
+ | volume = 1
+ | year = 1997
+ | pages = 276–279
+  | isbn = 9056990497
+}}</ref> Given two tensors ''T''<sub>1</sub>&isin;Sym<sup>''k''<sub>1</sub></sup>(''V'') and ''T''<sub>2</sub>&isin;Sym<sup>''k''<sub>2</sub></sup>(''V''), we use the symmetrization operator to define:
+:<math>T_1\odot T_2 = \operatorname{Sym}(T_1\otimes T_2)\quad\left(\in\operatorname{Sym}^{k_1+k_2}(V)\right).</math>
+It can be verified (as is done by Kostrikin and Manin<ref name="Kostrikin1997" />) that the resulting product is in fact commutative and associative. In some cases the operator is not written at all: ''T''<sub>1</sub>''T''<sub>2</sub> = ''T''<sub>1</sub><math>\odot</math>''T''<sub>2</sub>.
+In some cases an exponential notation is used:
+:<math>v^{\odot k} = \underbrace{v \odot v \odot \cdots \odot v}_{k\text{ times}}=\underbrace{v \otimes v \otimes \cdots \otimes v}_{k\text{ times}}=v^{\otimes k}.</math>
+Where ''v'' is a vector.
+Again, in some cases the '<math>\odot</math>' is left out:
+:<math>v^k=\underbrace{v\,v\,\cdots\,v}_{k\text{ times}}=\underbrace{v\odot v\odot\cdots\odot v}_{k\text{ times}}.</math>
+==Decomposition==
+In analogy with the theory of [[symmetric matrix|symmetric matrices]], a (real) symmetric tensor of order 2 can be "diagonalized".  More precisely, for any tensor ''T''&nbsp;&isin;&nbsp;Sym<sup>2</sup>(''V''), there are an integer ''r'', non-zero unit vectors ''v''<sub>1</sub>,...,''v''<sub>''r''</sub>&nbsp;&isin;&nbsp;''V'' and weights ''&lambda;''<sub>1</sub>,...,''&lambda;''<sub>''r''</sub> such that
+:<math>T = \sum_{i=1}^r \lambda_i \, v_i\otimes v_i.</math>
+The minimum number ''r'' for which such a decomposition is possible is the (symmetric) rank of ''T''.  The vectors appearing in this minimal expression are the ''[[principal axes]]'' of the tensor, and generally have an important physical meaning. For example, the principal axes of the [[inertia tensor]] define the [[Poinsot's ellipsoid]] representing the moment of inertia. Also see [[Sylvester's law of inertia]].
+For symmetric tensors of arbitrary order ''k'', decompositions
+:<math>T = \sum_{i=1}^r \lambda_i \, v_i^{\otimes k}</math>
+are also possible.  The minimum number ''r'' for which such a decomposition is possible is the ''symmetric'' [[Tensor_(intrinsic_definition)#Tensor_rank|rank]] of ''T''.<ref name="Comon2008" />  For second order tensors this corresponds to the rank of the matrix representing the tensor in any basis, and it is well-known that the maximum rank is equal to the dimension of the underlying vector space. However, for higher orders this need not hold: the rank can be higher than the number of dimensions in the underlying vector space.  The [[higher-order singular value decomposition]] of a symmetric tensor is a special decomposition of this form <ref name="Comon2008">{{Cite doi|10.1137/060661569}}</ref> (often called the [[CP decomposition|canonical decomposition]].)
+==See also==
+* [[antisymmetric tensor]]
+* [[Ricci calculus]]
+* [[Schur polynomial]]
+* [[symmetric polynomial]]
+* [[transpose]]
+* [[Young symmetrizer]]
+==Notes==
+<references/>
+==References==
+* {{citation|first = Nicolas|last=Bourbaki|authorlink=Nicolas Bourbaki | title = Elements of mathematics, Algebra I| publisher = Springer-Verlag | year = 1989|isbn=3-540-64243-9}}.
+* {{citation|first = Nicolas|last=Bourbaki|authorlink=Nicolas Bourbaki | title = Elements of mathematics, Algebra II| publisher = Springer-Verlag | year = 1990|isbn=3-540-19375-8}}.
+* {{Citation | last1=Greub | first1=Werner Hildbert | title=Multilinear algebra | publisher=Springer-Verlag New York, Inc., New York | series=Die Grundlehren der Mathematischen Wissenschaften, Band 136 | id={{MathSciNet | id = 0224623}} | year=1967}}.
+* {{Citation | last1=Sternberg | first1=Shlomo | author1-link=Shlomo Sternberg | title=Lectures on differential geometry | publisher=Chelsea | location=New York | isbn=978-0-8284-0316-0 | year=1983}}.
+==External links==
+* Cesar O. Aguilar, ''[http://www.mast.queensu.ca/~cesar/math_notes/dim_symmetric_tensors.pdf The Dimension of Symmetric k-tensors]''
+{{tensors}}
+[[Category:Tensors]]

Rational quadratic covariance function: Difference between revisions

Latest revision as of 08:16, 8 January 2013

Contents

Definition

Examples

Symmetric part of a tensor

Symmetric product

Decomposition

See also

Notes

References

External links

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Rational quadratic covariance function: Difference between revisions

Latest revision as of 08:16, 8 January 2013

Definition

Examples

Symmetric part of a tensor

Symmetric product

Decomposition

See also

Notes

References

External links

Navigation menu

Search