Homework #4, due Feb 26th:
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<li>1.7 #8,15,16,17,23
<li>2.1 #2,5,6,9,10,11
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1.7 #8. Let B = k-element subsets of A, and let S_A act on B
in the obvious way.
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a. To show it's an action, we need to check two things:
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<li> 1 . {a_1,...,a_k} = {1.a_1, ..., 1.a_k} = {a_1,...,a_k}. So the 
identity does act as the identity.
<li> (gh) . {a_1,...,a_k} = {(gh).a_1, ..., (gh).a_k} = {g.h.a_1, ...}
= g.{h.a_1,...}