Refined enumeration of two-rowed set-valued standard tableaux via two-coloured Motzkin paths

Abstract: We derive formulae for the number of set-valued standard tableaux of two-rowed shapes, keeping track of the total number of entries, the number of entries in the first row, and the number of entries in the second row. Key in the proofs is a bijection with two-coloured Motzkin paths followed by generating function computations and coefficient extraction helped by the Lagrange inversion formula.