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Nonlinear objectives, nonlinear constraints, non-convex objective, non-convex feasible region.

21 votes
Accepted

Prove that this expression is greater than 1/2

Let $$f(x,y):=4x^{2}+4y^{2}-4xy-4y+1 + \frac{4}{\pi^2}\Bigl( \sin^{2}(\pi x)+ \sin^{2}(\pi y) + \sin^{2}(\pi y-\pi x) \Bigr).$$ I will show that $$\min_{0\leq x\leq y\leq 1}f(x,y)=\min_{0\leq x\leq 1/ …
GH from MO's user avatar
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11 votes
Accepted

Min problem on integers

Let us denote $$\sigma_\ell:=\sum_{i=1}^\ell q_i\qquad\text{and}\qquad\tau_\ell:=\sum_{i=\ell+1}^s\frac{1}{q_i}.$$ Then $$\prod_{\ell=1}^{s-1}\left(\frac{q_\ell}{q_{\ell+1}}\cdot\frac{\sigma_{\ell+1}} …
GH from MO's user avatar
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12 votes

Smallest root of a degree 3 polynomial

This proof is similar to River Li's, but it requires no machine calculation. Also, we will show more, namely that the original polynomial has three real roots, the smallest of which is at most $$u:=\f …
GH from MO's user avatar
  • 105k
3 votes

Does this polynomial have a real zero less than or equal to $1/2$?

I will show more, namely that the polynomial in question has three real roots, the smallest of which is at most $$d:=\frac{5-\sqrt{15}}{5}=0.22540333\dotsc$$ This estimate is sharp, because in case th …
GH from MO's user avatar
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