f(x) = { x2 if x is rational
{ 0 if x is irrational
This function is related to the Dirichlet function, which is equal to 0 for rationals and 1 for irrationals. The Dirichlet function is never continuous, but here, f(x) is continuous at 0 and only 0.
The particular function I chose is also differentiable only at the single point x=0.
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