We present a single operation for constructing skew diagrams whose
corresponding skew Schur functions are equal. This combinatorial operation
naturally generalises and unifies all results of this type to date. Moreover,
our operation suggests a closely related condition that we conjecture is
necessary and sufficient for skew diagrams to yield equal skew Schur functions.Comment: 34 pages, 2 figures. Minor changes. Final version, to appear in
Transactions of the AM