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Leon van Wyk

Publications and source records attributed to Leon van Wyk.

13 recordsLinked to original sources
arXiv2024

The structure of subalgebras of full matrix algebras over a field satisfying the identity [x_1, y_1][x_2, y_2] ... [x_q, y_q] = 0

A subalgebra of the full matrix algebra Mn(K), K a field, satisfying the identity [x1, y1][x2, y2]...[xq, yq] = 0 is called a Dq subalgebra of Mn(K). In the paper we deal with the structure, conjugation and isomorphism problems of maximal Dq subalgebras of Mn(K). We show that a maximal Dq subalgebra A of Mn(K) is conjugated with a block triangular subalgebra of Mn(K) with maximal commutative diagonal blocks. By analysis of conjugations, the sizes of the obtained diagonal blocks are uniquely determined. It reduces the problem of conjugation of maximal Dq subalgebras of Mn(K) to the analogous problem in the class of commutative subalgebras of Mn(K). Further examining conjugations, in case A is contained in the upper triangular matrix algebra Un(K), we prove that A is already in a block triangular form. We consider the isomorphism problem in a certain class of maximal Dq subalgebras of Mn(K) which contain all Dq subalgebras of Mn(K) with maximum dimension. In case K is algebraically closed, we invoke Jacobson's characterization of maximal commutative subalgebras of Mn(K) with maximum (K-)dimension to show that isomorphic subalgebras in this class are already conjugated. To illustrate it, we invoke results from [17] and find all isomorphism (equivalently conjugation) classes of Dq subalgebras of Mn(K) with maximum possible dimension, in case K is algebraically closed.

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arXiv2021

Lie properties in associative algebras

Let K be a field, then we exhibit two matrices in the full nxn matrix algebra M_{n}(K) which generate M_{n}(K) as a Lie K-algebra with the commutator Lie product. We also study Lie centralizers of a not necessarily commutative unitary algebra and obtain results which we hope will eventually be a step in the direction of, firstly, proving that a Lie-nilpotent K-subspace (or a sub Lie K-algebra) of a finite-dimensional associative algebra over K of index k (say) generates a Lie-nilpotent associative subalgebra of much higher nilpotency index, and secondly, in the light of the sharp upper bound for the maximum (K-)dimension of a Lie-nilpotent K-subalgebra of M_{n}(K) of index k (obtained earlier), finding an upper bound for the maximum dimension of a Lie-nilpotent (of index k) sub Lie K-algebra of M_{n}(K). Finally, the constructive elementary proof of the Skolem-Noether theorem for the matrix algebra M_{n}(K) (appeared in the American Math. Monthly), in conjunction with the well-known characteization of Lie automorphisms of M_{n}(K) (if the characteristic of K is different from 2 and 3) in terms of, amongst others, automorphisms and anti-automorhisms of M_{n}(K), leads us to a unifying approach to constructively describe automorphisms and anti-automorphisms of M_{n}(K).

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arXiv2019

A power Cayley-Hamilton identity for nxn matrices over a Lie nilpotent ring of index k

For an nxn matrix A over a Lie nilpotent ring R of index k, we prove that an invariant "power" Cayley-Hamilton identity of degree (n^2)2^{k-2} holds. The right coefficients are not uniquely determined by A, and the cosets lambda_i+D, with D the double commutator ideal R[[R,R],R]R of R, appear in the so-called second right characteristic polynomial of the natural image of A in the nxn matrix ring M_{n}(R/D) over the factor ring R/D.

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arXiv2015

On Lie nilpotent rings and Cohen's Theorem

We study certain (two-sided) nil ideals and nilpotent ideals in a Lie nilpotent ring R. Our results lead us to showing that the prime radical rad(R) of R comprises the nilpotent elements of R, and that if L is a left ideal of R, then L+rad(R) is a two-sided ideal of R. This in turn leads to a Lie nilpotent version of Cohen's theorem, namely if R is a Lie nilpotent ring and every prime (two-sided) ideal of R is finitely generated as a left ideal, then every left ideal of R containing the prime radical of R is finitely generated (as a left ideal). For an arbitrary ring R with identity we also consider its so-called n-th Lie center Z_n(R), which is a Lie nilpotent ring of index n. We prove that if C is a commutative submonoid of the multiplicative monoid of R, then the subring of R generated by the union of Z_n(R) and C is also Lie nilpotent of index n.

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arXiv2014

Matrix representations of finitely generated Grassmann algebras and some consequences

We prove that the m-generated Grassmann algebra can be embedded into a 2^{m-1}x2^{m-1} matrix algebra over a factor of a commutative polynomial algebra in m indeterminates. Cayley-Hamilton and standard identities for nxn matrices over the m-generated Grassmann algebra are derived from this embedding. Other related embedding results are also presented.

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arXiv2012

Jordan Derivations and Antiderivations of Generalized Matrix Algebras

Let $\mathcal{G}=[A & M N & B]$ be a generalized matrix algebra defined by the Morita context $(A, B,_AM_B,_BN_A, Φ_{MN}, Ψ_{NM})$. In this article we mainly study the question of whether there exist proper Jordan derivations for the generalized matrix algebra $\mathcal{G}$. It is shown that if one of the bilinear pairings $Φ_{MN}$ and $Ψ_{NM}$ is nondegenerate, then every antiderivation of $\mathcal{G}$ is zero. Furthermore, if the bilinear pairings $Φ_{MN}$ and $Ψ_{NM}$ are both zero, then every Jordan derivation of $\mathcal{G}$ is the sum of a derivation and an antiderivation. Several constructive examples and counterexamples are presented.

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arXiv2011

The zero-level centralizer in endomorphism algebras

For an endomorphism f\inEnd{M) of a left R-module M we investigate the structure and the polynomial identities of the zero-level centralizer Cen_0(f) and the factor Cen(f)/Cen_0(f). A double zero-centralizer theorem for Cen_0(Cen_0(f)) is also formulated.

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arXiv2009

Algebras generated by two quadratic elements

Let K be a field of any characteristic and let R be an algebra generated by two elements satisfying quadratic equations. Then R is a homomorphic image of F=K for suitable a,b,c,d in K. We establish that F can be embedded into the 2x2 matrix algebra M_2(E[t]) with entries from the polynomial algebra E[t] over the algebraic closure E of K and that F and M_2(E) satisfy the same polynomial identities as K-algebras. When the quadratic equations have double zeros, our result is a partial case of more general results by Ufnarovskij, Borisenko and Belov from the 1980's. When each of the equations has different zeros, we improve a result of Weiss, also from the 1980's.

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arXiv2009

Centralizers in endomorphism rings

We prove that the centralizer Cen(f) in Hom_R(M,M) of a nilpotent endomorphism f of a finitely generated semisimple left R-module M (over an arbitrary ring R) is the homomorphic image of the opposite of a certain Z(R)-subalgebra of the full m x m matrix algebra M_m(R[z]), where m is the dimension (composition length) of ker(f). If R is a local ring, then we provide an explicit description of the above Cen(f). If in addition Z(R) is a field and R/J(R) is finite dimensional over Z(R), then we give a formula for the Z(R)-dimension of Cen(f). If R is a local ring, f is as above and g is an arbitrary element of Hom_R(M,M), then we give a complete description of the containment Cen(f) in Cen(g) in terms of an appropriate R-generating set of M. Using our results about nilpotent endomorphisms, for an arbitrary (not necessarily nilpotent) linear map f in Hom_K(V,V) of a finite dimensional vector space V over a field K we determine the PI-degree of Cen(f) and give other information about the polynomial identities of Cen(f).

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arXiv2007

Subrings which are closed with respect to taking the inverse

Let S be a subring of the ring R. We investigate the question of whether S intersected by U(R) is equal to U(S) holds for the units. In many situations our answer is positive. There is a special emphasis on the case when R is a full matrix ring and S is a structural subring of R defined by a reflexive and transitive relation.

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