42.12 Sum and Intersection of Pcgs

computes the sum and intersection of the lists n and u whose elements form modulo pcgs induced by parentpcgs for two subgroups modulo a kernel given by kerpcgs. If kerpcgs is a tail if the parent-pcgs it is sufficient to give the starting depth, this can be more efficient than to construct an explicit pcgs. The factor group modulo kerpcgs generated by n must be elementary abelian and normal under u.

The function returns a record with components

sum

Elements that form a modulo pcgs for the span of both subgroups.

intersection

Elements that form a modulo pcgs for the intersection of both subgroups.

factorization

A function that retruns for an element x in the span of sum a record with components u and n that give its decomposition.

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