Singular ideals over arbitrary fields for the cyclic-headed snakes

Abstract

We study the Steinberg algebras with coefficients in an arbitrary field K for the cyclic-headed snake groupoids, which are basic examples of non-Hausdorff groupoids. We are particularly interested in elements of this algebra that are no longer continuous, known as singular functions. These functions form an ideal, which may contain proper subsets that are themselves ideals of the Steinberg algebra. We provide three conditions under which such 'proper subset' ideals exist: first, when the number of heads of the snake divides the characteristic of the base field; second, when the base field is of non-prime characteristic; and third, when certain cyclotomic polynomials split over the base field. We also show the existence of many further subset ideals not covered by these conditions. We fully explore the cases of the two- and three-headed snakes. In the three-headed snake, we prove that the ideal of singular functions properly contains non-zero ideals of the Steinberg algebra if, and only if, the base field K is a splitting field of x^2 + x + 1, the third cyclotomic polynomial. Consequently, there are always proper subset ideals when K has characteristic a prime not congruent to -1 mod 3.

20 pages, comments welcome