Consider a functional, what is meant by a minimal sequence consistent of 'piecewise affine functions'?
What is an affine function?
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pde
functional-equations
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0@AlexisZorbas Affine functions are those of the form $f(x)=ax+b$ for some constants $a$ and $b$. Of course, $\arctan$ is not of that form. – 2015-06-12