Enumerative Results on the Schr\"oder Pattern Poset

The set of Schr\"oder words (Schr\"oder language) is endowed with a natural partial order, which can be conveniently described by interpreting Schr\"oder words as lattice paths. The resulting poset is called the Schr\"oder pattern poset. We find closed formulas for the number of Schr\"oder words covering/covered by a given Schr\"oder word in terms of classical parameters of the associated Schr\"oder path. We also enumerate several classes of Schr\"oder avoiding words (with respect to the length), i.e. sets of Schr\"oder words which do not contain a given Schr\"oder word.