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for questions involving inequalities, upper and lower bounds.

10 votes

An Linear Algebra Inequality

If you replace determinants by traces, then this inequality is just Cauchy-Schwarz for the inner product $(X,Y)=\mathop{\mathrm{tr}}(XY^T)$ on the space of matrices. Now, we just have to recall that d …
Vladimir Dotsenko's user avatar
4 votes

Show that $(\sum_{k=1}^{n}x_{k}\cos{k})^2+(\sum_{k=1}^{n}x_{k}\sin{k})^2\le (2+\frac{n}{4})\...

It is a bit long for a comment. Your question is about the matrix $A=(\cos((i-j)))_{i,j=1\ldots n}$, specifically, the maximum of the quadratic form $q(x)=(Ax,x)$ on the subset $M_+$ of the unit sp …
Vladimir Dotsenko's user avatar
3 votes

A property of unimodal sequences

For $k=0$ (a decreasing sequence), the proof is more or less the key idea that makes Leibniz convergence test work, for instance. Since the total alternating sum is zero, you are always reduced to the …
Vladimir Dotsenko's user avatar