For a permutation w in the symmetric group S₍, let L (w) denote the simple highest weight module in the principal block of the BGG category O for the Lie algebra sl₍ (C). We first prove that L (w) is Kostant negative whenever w consecutively contains certain patterns. We then provide a complete answer to Kostant's problem in type A₆ and show that the indecomposability conjecture also holds in type A₆, that is, applying an indecomposable projective functor to a simple module outputs either an indecomposable module or zero.
Mazorchuk et al. (Mon,) studied this question.
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