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# functional equation

A *functional equation* is an equation whose unknowns are functions.

$f(x+y)=f(x)+f(y),\quad f(x\cdot y)=f(x)\cdot f(y)$ are examples of such equations. The systematic study of these didn’t begin before the 1960’s, although various mathematicians have been studying them before, including Euler and Cauchy just to mention a few.

Functional equations appear many places, for example, the gamma function and Riemann’s zeta function both satisfy functional equations.

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## Mathematics Subject Classification

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