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Questions where the notion matrix has an important or crucial role (for the latter, note the tag matrix-theory for potential use). Matrices appear in various parts of mathematics, and this tag is typically combined with other tags to make the general subject clear, such as an appropriate top-level tag ra.rings-and-algebras, co.combinatorics, etc. and other tags that might be applicable. There are also several more specialized tags concerning matrices.

1 vote
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Eigenvalues of sum of two anti-commuting matrices

Suppose for simplicity's sak that $A$ and $B$ are diagonalizable over $\mathbb{R}$ and are non-singular. Let $V_a$ be the $a$-eigenspace of $A$. Then by anti-commutativity, we find $BV_a\subseteq V_{ …
Robin Chapman's user avatar
1 vote
Accepted

Operation of GL_n(Z/bZ)

Any transformation $$(v_1,\ldots,v_n)\mapsto (v_1,\ldots,v_{j-1},v_j+av_k,v_{j+1},\ldots,v_n)$$ for $j\ne k$ is achievable by means of some such matrix. It suffices to reduce an admissible vector to $ …
Robin Chapman's user avatar
4 votes
Accepted

Invariant quadratic forms of irreducible representations

There are certainly examples over $k=\mathbb{Q}$ where $\dim T\ge2$. Let's take the cyclic group $G$ of order $5$ and the representation space $$V=\{(a_0,\ldots,a_4)\in\mathbb{Q}^5:a_0+\cdots +a_4=0\} …
Robin Chapman's user avatar
3 votes

Matrix Conjugates over Finite Fields

This occurs if and only if the matrices $Q^r$ and $Q^s$ are conjugate. This is the case if and only if these matrices are conjugate over the algebraic closure of $\mathbb{F}_p$. …
Robin Chapman's user avatar
3 votes

Binary matrices with constant row and column sums

See this paper by Canfield and McKay. As the title suggests it focuses on the asymptotic enumeration, but it has lots of useful references. Added A simple arithmetic construction that realizes all pos …
Robin Chapman's user avatar
12 votes

Maximum determinant of $\{0,1\}$-valued $n\times n$-matrices

I shouldn't expect there to be exact results; compare the similar problem with matrices with entries $\pm1$. … The determination for which $n$ Hadamard matrices exist still resists proof. …
Robin Chapman's user avatar
12 votes
Accepted

Parametrization of O(3)

This parameterizes all such matrices once each. …
Robin Chapman's user avatar