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True or false: the mixed partial derivatives of every function of two variables are equal.

  • A.False, continuity of the mixed derivatives being required
  • B.True, the order of differentiation never mattering
  • C.True, provided the function is defined on the whole plane
  • D.True, whenever the first derivatives both exist

Answer: A

Clairaut's theorem gives equality only when the mixed derivatives are continuous at the point. Counterexamples exist in which \( f_{xy} \) and \( f_{yx} \) differ at the origin, both being discontinuous there. Mere existence of the first derivatives, or a domain covering the plane, is not enough

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