Reversing Palindromic Enumeration in Rank Two Free Groups

Abstract

The Gilman-Maskit algorithm for determining the discreteness or non-discreteness of a two-generator subgroup of $\mathrm{PSL}_2\mathbb{R}$ terminates with a pair of generators that are Farey words. The Farey words are primitive words that are indexed by rational numbers and infinity. The E-words, primitive words with palindromic product forms, are also indexed by rational numbers and infinity. We show a modification of the Gilman-Maskit algorithm so that the stopping generators are E-words.