(after Fubini, Guido 1879-1943) Theorem of integral calculus which, in general, states that the order of integration of a multiple integral does not affect the integral's result. For example, in integrating a well-defined function f(x,y) over a rectangle defined by {a<=x<=b; c<=y<=d}, it doesn't matter whether the internal nested integral is taken over {a,b} with respect to x, or over {c,d} with respect to y. This technique often allows difficult integrals in certain forms to be transformed into forms which may be much easier (or simply possible) to evaluate.