Commutators from a hyperplane of matrices

Denote by $M_n(K)$ the algebra of $n$ by $n$ matrices with entries in the field $K$. A theorem of Albert and Muckenhoupt states that every trace zero matrix of $M_n(K)$ can be expressed as $AB-BA$ for some pair $(A,B)$ of matrices of $M_n(K)$. Assuming that $n>2$ and that $K$ has more than 3 elements, we prove that the matrices $A$ and $B$ can be required to belong to an arbitrary given hyperplane of $M_n(K)$.Comment: 20 page

On Gerstenhaber's theorem for spaces of nilpotent matrices over a skew field

Let K be a skew field, and K_0 be a subfield of the central subfield of K such that K has finite dimension q over K_0. Let V be a K_0-linear subspace of n by n nilpotent matrices with entries in K. We show that the dimension of V is bounded above by q n(n-1)/2, and that equality occurs if and only if V is similar to the space of all n by n strictly upper-triangular matrices over K. This generalizes famous theorems of Gerstenhaber and Serezhkin, which cover the special case K=K_0.Comment: 17 pages (final version as will appear in Linear Algebra and its Applications

Primitive spaces of matrices with upper rank two over the field with two elements

For fields with more than $2$ elements, the classification of the vector spaces of matrices with rank at most $2$ is already known. In this work, we complete that classification for the field $\mathbb{F}_2$. We apply the results to obtain the classification of triples of locally linearly dependent operators over $\mathbb{F}_2$, the classification of the $3$-dimensional subspaces of $\text{M}_3(\mathbb{F}_2)$ in which no matrix has a non-zero eigenvalue, and the classification of the $3$-dimensional affine spaces that are included in the general linear group $\text{GL}_3(\mathbb{F}_2)$.Comment: 43 page

Large affine spaces of matrices with rank bounded below

Let K be an arbitrary (commutative) field with at least three elements, and let n, p and r be positive integers with r<=min(n,p). In a recent work, we have proved that an affine subspace of M_{n,p}(K) containing only matrices of rank greater than or equal to r must have a codimension greater than or equal to (r+1)r/2. Here, we classify, up to equivalence, these subspaces with the minimal codimension (r+1)r/2. This uses our recent classification of the affine subspaces of M_r(K) contained in GL_r(K) and which have the maximal dimension r(r-1)/2.Comment: 27 pages (from v2: a minor error has been corrected, the proofs are more detailed