Unary boundary complexes for pure string endpoints #
A pure-positive string which is maximal at its right endpoint but has a left hook has a one-middle Butler--Ringel boundary complex. The kernel of the left-hook projection is the positive arm before its first negative letter; its inclusion is a maximal right cohook. This file constructs that kernel word, proves the resulting coordinate sequence short exact, and proves both differentials irreducible in the finite module category.
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- kernel : Word R
- cohook : self.kernel.CohookExtension left.toLeftPositiveBoundaryExtension.result
- cutoff : length R self.kernel + 1 = left.toLeftPositiveBoundaryExtension.steps
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Regard a right hook on C as a left hook on the reversed word.
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Extending a word at its left endpoint preserves a peak at its right endpoint.
Construct the cohook kernel of a pure-positive left-hook projection when the hooked result is maximal at its right endpoint.
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The kernel data in the common one-sided case where the original pure word is already maximal at its right endpoint.
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The unary finite complex whenever the pure-positive hook result is a peak at its right endpoint.