Fubini's theorem, from 1907, expresses integration with respect to a product measure in terms of iterated integrals. The simpler version of this theorem for multiple Riemann integrals was used long before Fubini was around and of course was not known by his name. Nowadays it is common for the relation between multiple Riemann integrals and iterated integrals to be called Fubini's theorem in books.
A colleague of mine asked me when the label "Fubini's theorem" was first applied to this theorem about multiple Riemann integrals. (He considers it something of a travesty to use Fubini's name for this result in multivariable calculus books, where there is no measure theory content. As an example, in the 4th edition of Calculus (1990) by Larson, Hostetler, and Edwards the authors write "The following theorem was proved by the Italian mathematician Guido Fubini" and then they give a theorem on double integrals of continuous functions which certainly was not proved by Fubini.) I found this theorem does not have Fubini's name in some calculus and analysis books written decades ago: Whittaker and Watson's Modern Analysis (4th ed., 1927), Volume II of Apostol's Calculus (1962), Rudin's Principle of Mathematical Analysis (3rd ed., 1964), Thomas's Calculus and Analytic Geometry (4th ed., 1969), Bers's Calculus (1969), Loomis's Calculus (1974), Sherman Stein's Calculus and Analytic Geometry (2nd ed., 1977), George Simmons's Calculus with Analytic Geometry (1985), Marsden and Weinstein's Calculus III (1985), and Leithold's The Calculus with Analytic Geometry (5th ed., 1986). They all call this result something like "the theorem on iterated integrals".
I found the name "Fubini's theorem" used for multiple Riemann integrals in Spivak's Calculus on Manifolds (1965). Does anyone know of an earlier usage of the label "Fubini's theorem" for multiple Riemann integrals?
Best Answer
There is a note by R. T. Seeley in the American Mathematical Monthly from 1961 (vol. 68, pp. 56-57) titled Fubini Implies Leibniz Implies $F_{yx}=F_{xy}$. The note can be found in Selected Papers on Calculus (1969), edited by a committee chaired by Tom Apostol. I quote from the second paragraph:
"The note first assumes a simple form of Fubini's theorem for Riemann integrals (A), uses this to prove Leibniz's rule (B), and uses this in turn to prove one of the stronger forms of the theorem on mixed partials (C). It then states the corresponding results for Lebesgue integrals; the proofs remain the same."