The symmetry of second derivatives refers to a result in multivariable calculus often associated with functions of several variables. Specifically, if a function \( f \) has continuous second partial derivatives, then the mixed second derivatives are equal.
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When the second partial derivatives are continuous in a neighborhood, the order of the mixed partial derivatives can be interchanged. Expressing a rectangular finite difference as either iterated integral by the fundamental theorem of calculus and shrinking the rectangle proves the equality. Mere existence of both derivatives does not imply this symmetry.