gh-37757: dft for symmetric group algebra when p|n!
The DFT for the SGA throws a ZeroDivisionError when p|n. Uses the
idempotents as computed in [Murphy '83] to build the Pierce
decomposition of the symmetric group algebra over a finite field. There
is a homomorphic projection onto each block via v |--> v*e_i. The change
of basis matrix is the modular Fourier transform, and works even when
p|n.
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URL: https://github.com/sagemath/sage/pull/37757
Reported by: Jackson Walters
Reviewer(s): grhkm21, Jackson Walters, Sebastian A. Spindler, Travis Scrimshaw
