Dense 2-generator subsemigroups of $2\times 2$ matrices

Abstract

We show that the semigroup of real linear fractional transformations on a proper subinterval of the real line does not admit any 2-generator dense subsemigroups, and then we construct a 3-parameter family of examples of 3-generator dense subsemigroups. We also construct an explicit example of a 2-generator dense subsemigroup of $2\times 2$ real matrices. In the complex case, we prove the existence of uncountably many 2-generator dense subsemigroups.