When we can pull expected value into the function to get inequality?

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Suppose we have $\mathbb{E}(f(x))$, what are the assumptions on $f$ that enable us to have $\mathbb{E}(f(x)) \leq f(\mathbb{E}(x))$?

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if $f$ is a concave function, then $-f$ is convex.

By Jensen's inequality,

$$-f(E(X))\le E[-f(X)]$$

Multiply $-1$ on both sides and we have

$$f(E(X)) \ge E[f(X)]$$