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Questions about the properties of vector spaces and linear transformations, including linear systems in general.

12 votes
Accepted

Vector of integers such that almost all dot products are positive

Sure, this is true. By the comment of Joseph Van Name we basically want a hyperplane passing through $0$ separating all the permutations of our numbers viewed as vectors in $\mathbb{R}^n$ from $(x_1, …
Aleksei Kulikov's user avatar
2 votes

Positive linear recurrent sequence

Even more is true: for all linear recurrences either $|a_k| \le xr^k$ for some $x>0, r < 1$ or $\limsup |a_k| > 0$. Indeed, for any linear recurrence we have $a_k = \sum_{m = 1}^N b_m k^{c_m} d_m^k$ f …
Aleksei Kulikov's user avatar
4 votes
Accepted

An upper estimate for $|\det(A+B)|$

Okay, here is the proof that $C(n) = \frac{2^{n-1}}{\sqrt{n}^n}$. Firstly, it is enough to find best constant $c(n)$ in $|\det(A)| \le c(n)||A||^n$. Indeed, we have $$|\det(A+B)| \le c(n)||A+B||^n \l …
Aleksei Kulikov's user avatar