Tensors |
In General > s.a. tensor fields [tensor densities, calculus];
types of fiber bundles [tensor bundles].
* History: Tensors were first fully described
in the 1890s by Gregorio Ricci-Curbastro, with the help of his student Tullio Levi-Civita,
and they were given their name in 1898 by Woldemar Voigt, a German crystallographer, who
was studying stresses and strains in non-rigid bodies.
$ Def 1: (Cartan's point of view)
A (p, q)-tensor over a vector space V is a multilinear map from
p copies of V* and q copies of V to a field (in practice,
\(\mathbb R\) or \(\mathbb C\)),
T: V* × V* × ... V* × V × V × ... V → \(\mathbb R\).
$ Def 2: (Transformation point of view) A (p, q)-tensor over a vector space V is an object which, under a change of basis for V represented by the matrix A, transforms under
T' a.... bc... d = Aam ... Abn T m... np... q \((A^{-1})^p{}_c \ldots (A^{-1})^q{}_d\) .
* Special cases: If V
is real n-dimensional, tensors of type (p, q)
on V carry a representation of GL(n, \(\mathbb R\)).
* Symmetry properties:
A p-th order covariant (for example) tensor T has
the symmetry (or antisymmetry) defined by π ∈
Sp if πT
= T (or πT = (−1)sign(π) T),
where the action of π is defined by πT(v1,
..., vp):=
T(vπ(1),
..., vπ(p)).
@ Student guides:
Fleisch 11;
Battaglia & George AJP(13)jul [undergraduate].
@ And physics: Joshi 95;
Jeevanjee 11 [r PT(12)apr].
@ Other references: Olive & Auffray MMCS-a1301 [symmetry classes for even-order tensors];
Kissinger a1308 [abstract indices, categorical treatment].
> Online resources:
see Marcus Hanke's page;
MathWorld page.
Tensor Algebra and Operations
> s.a. Algebraic Geometry [decomposition of tensors];
computation [including symbolic manipulation].
* Idea: The set of tensors
of type (p, q) is a linear space, while the set of
all tensors forms an algebra with the operations of addition and tensor
product; Additional operations defined on it are contraction, trace, ...
* Identity: For any two rank-2
antisymmetric tensors Aab
and Bab in 4D, A B –
*A*B = \(1\over2\) delta A B.
$ Symmetrization: Given
a p-th order tensor T, the action of the symmetrization
operator A on it is
S T:= \(1\over p!\)∑π ∈ Sp πT .
$ Antisymmetrization: Given a p-th order tensor T, the action of the antisymmetrization operator A on it is
A T:= \(1\over p!\)∑π ∈ Sp (signature of π) πT ,
or, in components, (AT)i1,
..., ip:=
\(1\over p!\)εi1,
..., ipk1,
..., kp Tk1,
..., kp.
* With a metric:
@ References: Edgar & Höglund JMP(02)gq/01 [generalized Lovelock identity];
Portugal & Svaiter mp/01,
Manssur & Portugal IJMPC(02)mp/01 [symbolic manipulation].
Tensor Product between Tensors > s.a. metric tensor.
$ Def: Given, for example, the
two tensors u ∈ ⊗q
TxX and
θ ∈ ⊗p
T*x X,
their tensor product is the tensor T
= u ⊗ θ ∈ {⊗q
Tx X ⊗p
T*x X}, defined by
T(ω1, ..., ωq, v1, ..., vp):= u(ω1, ..., ωq) θ(v1, ..., vp) ,
or, using abstract index notation, T a... bc... d:= ua... b θc... d .
Tensor Product between Vector Spaces
$ Def: V ⊗ W:= {f
: V* × W* → \(\mathbb R\) (\(\mathbb C\)), f bilinear}, with
(af + bg) (ξ, η):= a f(ξ,
η) + b g(ξ, η).
* In practice: If {vi}
is a basis for V and {wm}
one for W, V ⊗ W:=
{f = ∑im cim
vi ⊗ wm |
cim ∈ \(\mathbb R\) (\(\mathbb C\))}.
* With Hilbert space structure:
If f = ∑im cim
vi ⊗ wm
and g = ∑im dim
vi ⊗ wm,
then \(\langle\)f, g\(\rangle\):= ∑ijnm
cim djn
\(\langle v_i, v_j \rangle_V\, \langle w_m, w_n \rangle_W\) .
@ Between Banach spaces: Grothendieck BSMSP(56) [tensor norms].
Generalizations > s.a. Holors; quantum
states [tensor network factorization]; tensor fields.
@ References:
Fernández et al AACA(01)mp/02 ["extensors"];
Gaete & Wotzasek PLB(06) [negative rank?];
Christandl & Zuiddam CC(18)-a1606 [tensor surgery].
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