What is the relationship between functions and expected values?

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Imagine a random variable $X$ and a function $f(x)$.

What properties must $f(x)$ have for $f(E[X]) = E[f(X)]$.

Example: Do we know the average kinetic energy of a set of particles, if we know the average speed?

Here, $f(v) = \dfrac12 m v^2$

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$f$ should be an affine function, i.e. $f(x)=ax+b$ for some $a, b$. This follows from linearity of integration.

In general, otherwise, the equality does not hold.