International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. B, ch. 1.3, pp. 2728

Let , . Then the function is called the tensor product of f and g, and belongs to . The finite linear combinations of functions of the form span a subspace of called the tensor product of and and denoted .
The integration of a general function over may be accomplished in two steps according to Fubini's theorem. Given , the functions exist for almost all and almost all , respectively, are integrable, and Conversely, if any one of the integrals is finite, then so are the other two, and the identity above holds. It is then (and only then) permissible to change the order of integrations.
Fubini's theorem is of fundamental importance in the study of tensor products and convolutions of distributions.