Young Tableaux, Multisegments, and PBW Bases

The crystals for finite dimensional representations of sln+1 can be realized using Young tableaux. The infinity crystal on the other hand is naturally realized using multisegments, and there is a simple description of each embedding B(λ) ,→ B(∞) in terms of these realizations. The infinity crystal is also parameterized by Lusztig’s PBW basis with respect to any reduced expression for w0. We give an explicit description of the unique crystal isomorphism from PBW bases to multisegments in the case where w0 = s1s2s3 · · · sns1 · · · s1s2s1, thus obtaining simple formulas for the actions of all crystal operators on this PBW basis. Our proofs use the fact that the twists of the crystal operators by Kashiwara’s involution also have simple descriptions in terms of multisegments, and a characterization of B(∞) due to Kashiwara and Saito. These results are to varying extents known to experts, but we do not think there is a self-contained exposition of this material in the literature, and our proof of the relationship between multisegments and PBW bases seems to be new.