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May 19, 2021 at 2:23 comment added Iiro Ullin You are totally right of course, I am just commenting that it's not necessarily totally obvious for someone who's asking this question in the first place.
May 19, 2021 at 0:25 comment added Piotr Hajlasz @IiroUllin It does. If $0\leq f\leq 1$, then the Bernstein polynomials satisfy the same estimate. $p\geq 0$ is obvious, because all terms in the definition of $p$ are nonnegative and then replacing $f(k/n)$ by the upper bound of $1$ you get the binomial formula for $((1-x)+x)^n$ which equals $1$ which is the upper bound for $p$. If I find time, I will add details.
May 18, 2021 at 23:50 comment added Iiro Ullin The answer is fitting, but perhaps does not fully address the $0\leq p\leq1$ constraint. So I would add that the reason that $p$ lies within the same bounds as $f$ can be most clearly seen via De Casteljau's construction of the corresponding polynomial approximation (convex hull property of Bezier curves).
May 18, 2021 at 21:35 vote accept Rahul Sarkar
May 18, 2021 at 21:28 history answered Piotr Hajlasz CC BY-SA 4.0