1.2.50 Type Zero One Tensor Definition
A Type Zero One Tensor is a rank-0 tensor with a single component, fundamental in tensor algebra for representing scalars in multilinear contexts.
Type Zero One Tensor Definition is the characterization of a tensor with no contravariant factors and exactly one covariant factor, the case in which the type reduces to . A type tensor is, by definition, a linear functional, or covector, so that the dual space of a vector space is itself already an instance of the tensor concept, standing as the covariant counterpart to ordinary vectors within the type-graded tensor algebra.
Formal Definition
Let be a vector space over a field , with dual space . Setting and in the general tensor product space
leaves no copies of and a single copy of , giving
so that a type tensor is precisely an element : a linear functional on , carrying a single covariant index and no contravariant index.
The Unary Multilinear Map Interpretation
Under the correspondence between tensors of type and multilinear maps taking covector and vector arguments, a type tensor corresponds directly to the map
sending a vector to the scalar , with no further reinterpretation needed: the definition of a type tensor as an element of and its definition as a linear functional on are one and the same, since is, by its own construction, the set of linear functionals on .
Transformation Law
The single index of a type tensor transforms covariantly: under a change of basis with matrix , the components transform directly using itself,
with no matrix inversion involved. This direct transformation, applied to a single index, is what gives the type case its name as the prototypical covariant tensor, and it is required precisely to keep the scalar value unchanged whenever both the covector's components and the vector's components are simultaneously rewritten in a new basis.
Order, Degree, and Arity
A type tensor has total order one, occupies the degree-one summand of the covariant tensor algebra built from alone, and has arity one when regarded as a multilinear map on a single vector argument. Its contravariant order is exactly zero and its covariant order is exactly one, so, as in the type case, every one of the standard classifying invariants — order, type, valence, and arity — agrees on this simplest nontrivial covariant example without ambiguity.
Distinction from Type (1, 0)
A type tensor is not interchangeable with a type tensor despite both having total order one, since the two obey inverse transformation laws under a change of basis. Only with additional structure on , most commonly a nondegenerate bilinear form such as a metric tensor, can a canonical correspondence between the two types be established, in which case the operation converting a type tensor into a type tensor is called lowering an index, and its inverse, applied in the other direction, is called raising an index.
Role Within Tensor Algebra
Type tensors are the building blocks from which every higher-order purely covariant tensor is assembled by repeated tensor product, since a decomposable tensor of type for is a product of individual type tensors, and every element of the full covariant tensor algebra is a finite sum of such products. Recognizing covectors as type tensors places the dual space within the same unified definitional framework used for tensors of every other type, confirming that linear functionals, alongside vectors, form one of the two elementary generating pieces from which the entire tensor algebra is constructed.