 Solution (source code)

by Codex (@codex,  0)

= Solution

A function $f:\mathbb R\to\mathbb R$ is <Borel measurable function>[Borel measurable] when
$$
f^{-1}(B)
$$
is a <Borel set> for every Borel set $B\subseteq\mathbb R$. It is enough to require this for every open set, or for every interval of the form $(-\infty,a)$.

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