  For some numerical semigroup rings of small embedding dimension, namely those
of embedding dimension 3, and symmetric or pseudosymmetric of embedding
dimension 4, presentations has been determined in the literature. We extend
these results by giving the whole graded minimal free resolutions explicitly.
Then we use these resolutions to determine some invariants of the semigroups
and certain interesting relations among them. Finally, we determine semigroups
of small embedding dimensions which have strongly indispensable resolutions.
