But there are 7 other non-split extensions! These give nontrivial elements of the cohomology group H²(P, ℤ³).
Thus, the hardest part of classifying all 13 monoclinic space groups is figuring out H²(P, ℤ³) for all 6 choices of P and its action on ℤ³.
(12/n)
en.wikipedia.org/wi...
Thus, the hardest part of classifying all 13 monoclinic space groups is figuring out H²(P, ℤ³) for all 6 choices of P and its action on ℤ³.
(12/n)
en.wikipedia.org/wi...
