Monday, 21 May 2012

co.combinatorics - Where do stable Kronecker coefficients live "in nature"?

Background:



For a partition lambda, let lambda[N]=(N|lambda|,lambda1,lambda2,lambda3,dots), also let chilambda be the corresponding irreducible character of the symmetric group S|lambda|. Even if lambda[N] is not a partition, we can make sense of chilambda[N] as a class function of SN using a determinant, and Murnaghan proved that there exist coefficients Gnulambda,mu (the stable, or reduced, Kronecker coefficients) such that



chilambda[N]chimu[N]=sumnuGnulambda,muchinu[N]



for all Nge0.



Question:



In a sense, the construction of lambda[N] is a bit ad hoc. Is there a more representation-theoretic way to define these coefficients? In particular, if |lambda|+|mu|=|nu|, then Gnulambda,mu coincides with the corresponding Littlewood-Richardson coefficient, so I am hopeful there is some connection. I am looking for an answer which addresses the following point: as I have defined these coefficients, it seems that the most accessible way to work with these coefficients is to use combinatorics. If I wanted to use tools from say, invariant theory or algebraic geometry, what is a more natural context for these coefficients to appear?

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