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

9 votes

Can the sum of two roots of unity be a root of unity?

True. For three terms $1+i-i=1$, all of which are $4^{th}$-root of $1$. For two terms you can also write $-\frac{1+\sqrt{3}i}{2}-\frac{1-\sqrt{3}i}{2}=-1$, all of which are $6^{th}$-root of $1$. And …
Loïc Teyssier's user avatar
14 votes
4 answers
3k views

Vandermonde matrix is totally positive

A totally positive matrix $M\in \mathcal{M}_{n\times m}(\mathbb R)$ is such that all of its minors of all sizes are positive. It is true that any Vandermonde matrix (with well-ordered positive entries …
Loïc Teyssier's user avatar