Diamond distances in Nottingham algebras

TL;DR

This paper studies Nottingham algebras, a class of graded Lie algebras, showing that the pattern of their diamonds can be classified by types and degrees, and proving a key degree difference property.

Contribution

It establishes that all diamonds in Nottingham algebras can be assigned types, and the degree difference between consecutive diamonds is always q-1, aiding classification efforts.

Findings

Diamonds past the first can be assigned types that classify the algebra.

The degree difference between consecutive diamonds is always q-1.

Classified Nottingham algebras where all diamonds have type infinity.

Abstract

Nottingham algebras are a class of just-infinite-dimensional, modular, $\mathbb{N}$-graded Lie algebras, which includes the graded Lie algebra associated to the Nottingham group with respect to its lower central series. Homogeneous components of a Nottingham algebra have dimension one or two, and in the latter case they are called diamonds. The first diamond occurs in degree $1$, and the second occurs in degree $q$, a power of the characteristic. Many examples of Nottingham algebras are known, in which each diamond past the first can be assigned a type, either belonging to the underlying field or equal to $\infty$. A prospective classification of Nottingham algebras requires describing all possible diamond patterns. In this paper we establish some crucial contributions towards that goal. One is showing that all diamonds, past the first, of an arbitrary Nottingham algebra $L$ can be assigned a type, in such a way that the degrees and types of the diamonds completely describe $L$. At the same time we prove that the difference in degrees of any two consecutive diamonds in any Nottingham algebra equals $q-1$. As a side-product of our investigation, we classify the Nottingham algebras where all diamonds have type $\infty$.