# Who first proved that $f_{xy} = f_{yx}$?

Who first proved the interchangeability of partial derivatives? I never see any reference in textbooks. This is not a trivial result.

• For analytic functions it is trivial. The problem is that the modern definition of function was only given in 19th century. Previously most mathematicians thought of "functions" as analytic functions. Jan 13, 2018 at 19:50
• @AlexandreEremenko What would be the contrast between the "modern definition of function" and "analytic functions"?
– Nat
Jan 16, 2018 at 2:38
• clairaut? schwarz? young?
– BCLC
Nov 29, 2021 at 16:15

It looks as if it was Euler who first proved it. See A note on the history of mixed partial derivatives, by Thomas James Higgins (Scripta Mathematica 7 (1940), pp. 59–62).

• Nice paper. One might add that (according to Euler himself) the first to put it in anything like the notation in the title was Fontaine (1738, p. 26). Clairaut (1742, footnote p. 294) discusses the independence of Euler’s, Fontaine’s, and his own proof. Jan 13, 2018 at 15:30
• One might also add that Lindelöf in 1867 criticized all proofs that had been given so far and that the first rigorous proof was subsequently presented by Schwarz in 1873. Jan 26, 2018 at 23:42
It was H.A. Schwarz who proved the theorem: If a function $f:\mathbb{R}^n \rightarrow\mathbb{R}^n$ is $m$ times differentiable and continuous, then the $m$th mixed derivatives are independent of the order.
• I think you should mention notation $f \in C^m$, not for the sake of notation per se, but to help the person who asked the question. Jan 20, 2018 at 6:21