Topological embeddings into transformation monoids

In this paper we consider the questions of which topological semigroups embed topologically into the full transformation monoid N-N or the symmetric inverse monoid I-N with their respective canonical Polish semigroup topologies. We characterise those topological semigroups that embed topologically into N-N and belong to any of the following classes: commutative semigroups, compact semigroups, groups, and certain Clifford semigroups. We prove analogous characterisations for topological inverse semigroups and IIN . We construct several examples of countable Polish topological semigroups that do not embed into N-N , which answer, in the negative, a recent open problem of Elliott et al. Additionally, we obtain two sufficient conditions for a topological Clifford semigroup to be metrizable, and prove that inversion is automatically continuous in every Clifford subsemigroup of N-N . The former complements recent works of Banakh et al.