Business I / Partial Derivatives
Practice question · True or false

Young's theorem guarantees fxy=fyxf_{xy} = f_{yx} for every function of two variables.

Hints
  1. What condition does the theorem attach?
  2. Continuity of the mixed partials.
Show the answer

False

Why

False. Equality holds when the mixed partials are continuous at the point. Counterexamples exist, the standard one built from xy(x2y2)/(x2+y2)xy(x^2-y^2)/(x^2+y^2) has fxy(0,0)fyx(0,0)f_{xy}(0,0) \neq f_{yx}(0,0), so the hypothesis is doing real work, even though it holds for essentially every function met in economics.

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