On the image of polynomials evaluated on incidence algebras: a counter-example and a solution

In this paper, we investigate the subset obtained by evaluations of a fixed multilinear polynomial on a given algebra. We provide an example of a multilinear polynomial, whose image is not a vector subspace; namely, the product of two commutators need not to be a subspace whenever evaluated on certain subalgebras of upper triangular matrices (the so-called incidence algebras). In the last part of the paper, given that the field is infinite, we reduce the problem of the description of the image of a polynomial evaluated on an incidence algebra to the study of evaluations of a certain family of polynomials on its Jacobson radical. In particular, we are able to describe the image of multilinear polynomials evaluated on the algebra of upper triangular matrices.