Representability of relatively free affine algebras over a Noetherian ring

TL;DR

This paper extends the hiking technique to nonhomogeneous polynomials to prove the representability of affine relatively free affine algebras over Noetherian rings, especially in positive characteristic.

Contribution

It develops a new approach using hiking for nonhomogeneous polynomials to establish representability over Noetherian rings, generalizing previous results.

Findings

Proves representability of affine relatively free PI algebras over Noetherian rings.

Extends hiking method to nonhomogeneous polynomials in positive characteristic.

Provides a complete proof using Noetherian induction.

Abstract

Over the years questions have arisen about T-ideals of (noncommutative) polynomials. But when evaluating a noncentral polynomial in subalgebras of matrices, one often has little control in determining the specific evaluations of the polynomial. One way of overcoming this difficulty in characteristic 0, is to reduce to multilinear polynomials and utilizing the representation theory of the symmetric group. But this technique is unavailable in characteristic $p>0$. An alternative method, which succeeds, is the process of ``hiking'' a polynomial, in which one specializes its indeterminates in several stages, to obtain a polynomial that contains Capelli polynomials, in order to get control on its evaluations. This method was utilized on homogeneous polynomials in the proof of Specht's conjecture for affine algebras over fields of positive characteristic. In this paper we develop hiking further to nonhomogeneous polynomials, to apply to the representability question. Kemer proved in 1988 that every affine relatively free PI algebra over an infinite field, is representable. In 2010, the first author of this paper proved more generally that every affine relatively free PI algebra over any commutative Noetherian unital ring is representable. We present a different, complete, proof, based on hiking nonhomogeneous polynomials, over finite fields. We then obtain the full result over a Noetherian commutative ring, using Noetherian induction on T-ideals. The bulk of the proof is for the case of a base field of positive characteristic. Here, whereas the usage of hiking is more direct than in proving Specht's conjecture, one must consider nonhomogeneous polynomials when the base ring is finite, which entails certain difficulties to be overcome. In the appendix we show how hiking can be adapted to prove the involutory versions, as well as various graded and nonassociative theorems.