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Nonlinear objectives, nonlinear constraints, non-convex objective, non-convex feasible region.
4
votes
Accepted
Proving an infinite norm minimization problem has finite support (non-convex p-norms)
If $p=1$, $N=1$ and $a_1=(1/2,2/3,3/4,4/5,\ldots)$, the infimum equals 1 and is not achieved on a finitely supported vector (moreover, it is not achieved at all).
However if $0<p<1$ and the minimize …
37
votes
Accepted
How to prove $e^x\left|\int_x^{x+1}\sin(e^t) \,\mathrm d t\right|\le 1.4$?
Integrate by parts:
\begin{align}
\int_x^{x+1}\sin(e^t)dt
& =\int_x^{x+1}e^{-t}d(-\cos(e^t)) \\
& =e^{-x}\cos e^x-e^{-x-1}\cos e^{x+1}-\int_x^{x+1}e^{-t}\cos e^{t}dt\\
& =e^{-x}\cos e^x-e^{-x-1}\cos e …
3
votes
Accepted
Maximizing the distance sum of some points inside a circle
Let 0 be the centre of your circle of radius $r$.
If $a=2$, we expand the brackets and write $\sum_{i,j} (p_i-p_j)^2=2n\sum p_i^2-2(\sum p_i)^2\leqslant 2nr^2$, with equality if and only if all points …
5
votes
Accepted
Bound on the sum of arguments
Let's prove that
$$
\arg \frac{1-zf(s-u)}{1-zf(s+u)}< \pi/2- \arg(1-z\bar{z}f(2u)).
$$
Then summing this up with an analogous inequality
$$
\arg \frac{1-zf(t+u)}{1-zf(t-u)}< \pi/2- \arg(1-z\bar{z}f(- …
8
votes
Accepted
Elementary inhomogeneous inequality for three non-negative reals
Denote $x^2=a^3,y^2=b^3,z^2=c^3$. By AM-GM we have $1+2xyz=1+(abc)^{3/2}+(abc)^{3/2}\geqslant 3\sqrt[3]{1\cdot (abc)^{3/2}\cdot (abc)^{3/2}}=3abc$, so LHS is not less then $$a^3+b^3+c^3+3abc\geqslant …
16
votes
Accepted
Maximization of a cubic form over the $14$-dimensional sphere
Put $x_{ij}=x_{ji}$ and consider the symmetriс matrix $A=(x_{ij})$ with zeros on diagonal. Then $\operatorname{tr} A=0$, $\operatorname{tr} A^2=2\sum_{i<j} x_{ij}^2=2$ is fixed, denote it $2=30 \alpha …