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From: Mitchell Hockley on 2 Jun 2010 06:01 Hi, Could anyone help me with this question (from Bostock's Intermediate Logic) "The theory of groups can be presented as having in its vocabulary just identity and a single two-place function f(x,y) which we write as 'x.y'. The usual laws for identity apply, and in addition these three axioms:" (A1) Axyz(x.(y.z) = (x.y).z) (A2) AxyEz(x = z.y) (A3) AxyEz(x = y.z) Prove: a = a.c |= c = c.c (Hint: Use Ez(c = z.a) Thanks for any help, Mitch. |