One of the original motivations for topological tensor products is the fact that tensor products of the spaces of smooth functions on do not behave as expected. There is an injection but this is not an isomorphism. For example, the function cannot be expressed as a finite linear combination of smooth functions in We only get an isomorphism after constructing the topological tensor product; i.e., This article first details the construction in the Banach space case. is not a Banach space and further cases are discussed at the end.
We shall use the notation from in this section. The obvious way to define the tensor product of two Banach spaces A and B is to copy the method for Hilbert spaces: define a norm on the algebraic tensor product, then take the completion in this norm. The problem is that there is more than one natural way to define a norm on the tensor product. If A and B are Banach spaces the algebraic tensor product of A and B means the tensor product of A and B as vector spaces and is denoted by. The algebraic tensor product consists of all finite sums where is a natural number depending on and and for When A and B are Banach spaces, a cross normp on the algebraic tensor product is a norm satisfying the conditions Here a′ and b′ are in the topological dual spaces of A and B, respectively, and p′ is the dual norm of p. The term reasonable crossnorm is also used for the definition above. There is a cross norm called the projective cross norm, given by where. It turns out that the projective cross norm agrees with the largest cross norm. There is a cross norm called the injective cross norm, given by where. Here A′ and B′ mean the topological duals of A and B, respectively. Note hereby that the injective cross norm is only in some reasonable sense the "smallest". The completions of the algebraic tensor product in these two norms are called the projective and injective tensor products, and are denoted by and When A and B are Hilbert spaces, the norm used for their Hilbert space tensor product is not equal to either of these norms in general. Some authors denote it by σ, so the Hilbert space tensor product in the section above would be A uniform crossnorm α is an assignment to each pair of Banach spaces of a reasonable crossnorm on so that if are arbitrary Banach spaces then for all operators and the operator is continuous and If A and B are two Banach spaces and α is a uniform cross norm then α defines a reasonable cross norm on the algebraic tensor product The normed linear space obtained by equipping with that norm is denoted by The completion of which is a Banach space, is denoted by The value of the norm given by α on and on the completed tensor product for an element x in is denoted by or A uniform crossnorm is said to be finitely generated if, for every pair of Banach spaces and every, A uniform crossnorm is cofinitely generated if, for every pair of Banach spaces and every, A tensor norm is defined to be a finitely generated uniform crossnorm. The projective cross norm and the injective cross norm defined above are tensor norms and they are called the projective tensor norm and the injective tensor norm, respectively. If A and B are arbitrary Banach spaces and α is an arbitrary uniform cross norm then
Tensor products of locally convex topological vector spaces
The topologies of locally convex topological vector spaces and are given by families of seminorms. For each choice of seminorm on and on we can define the corresponding family of cross norms on the algebraic tensor product and by choosing one cross norm from each family we get some cross norms on defining a topology. There are in general an enormous number of ways to do this. The two most important ways are to take all the projective cross norms, or all the injective cross norms. The completions of the resulting topologies on are called the projective and injective tensor products, and denoted by and There is a natural map from to If or is a nuclear space then the natural map from to is an isomorphism. Roughly speaking, this means that if or is nuclear, then there is only one sensible tensor product of and. This property characterizes nuclear spaces.
