The Combinatorics of A₂-webs

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The nonelliptic A₂-webs with k "+"s on the top boundary and 3n-2k "-"s on the bottom boundary combinatorially model the space Homsl₃(V⊗ ⁽³ⁿ⁻²ᵏ⁾, V⊗ ᵏ) of sl₃-module maps on tensor powers of the natural 3-dimensional sl₃-module V, and they have connections with the combinatorics of Springer varieties. Petersen, Pylyavskyy, and Rhodes showed that the set of such A₂-webs and the set of semistandard tableaux of shape (3ⁿ) and type {1²,,k²,k+1,, 3n-k} have the same cardinalities. In this work, we use the m-diagrams introduced by Tymoczko and the Robinson-Schensted correspondence to construct an explicit bijection, different from the one given by Russell, between these two sets. In establishing our result, we show that the pair of standard tableaux constructed using the notion of path depth is the same as the pair constructed from applying the Robinson-Schensted correspondence to a 321-avoiding permutation. We also obtain a bijection between such pairs of standard tableaux and Westbury's A₂ flow diagrams.