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11 votes
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Conditions for a power of a polynomial to have no negative coefficients

Marcus Michelen, Julian Sahasrabudhe, A characterization of polynomials whose high powers have non-negative coefficients, https://arxiv.org/abs/1910.06890
Alexandre Eremenko's user avatar
9 votes

Polynomials such that $|p(z)|\leq p(|z|)$

The result proved in the answer of @Terry Tao is actually due to Teichmuller, see, for example L. Ahlfors, Conformal invariants, section 3-4. For an interesting related result, see MR0344465 Boĭčuk, V …
Alexandre Eremenko's user avatar
1 vote
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Does every locally positive-definite function have a positive-definite extension?

This is true for $d=1$ (M. Krein) but not true for $d>1$ (W. Rudin). For a criterion of extension see O. Jorgensen, R. Niedzialomski, Extension of positive definite functions, J. Math. Anal. Appl. …
Alexandre Eremenko's user avatar
10 votes

Positivity of a one-variable rational function

One can show that all coefficients with sufficiently large index are positive. Indeed, using Maple, the pole of $f$ closest to the origin is: $a:=0.543689...>0,$ and the residue at this pole is $c:=-0 …
Alexandre Eremenko's user avatar