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Proof that $\exp(x)$ is the only function for which $f(x) = f'(x)$
I've learned from calculus that $Ce^x$ is a solution to the equation $f(x)=f'(x)$, where $C$ is a constant. However, in my Physics class the textbook takes it as the only solution to the equation without proof.
I wonder why there are no other solutions? I haven't taken the course of ODE, so I might need an elementary explanation~ Any help will be welcome~