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| + | I used what we proved in Ch 9 problem 7 to prove this. In that problem, we proved that if H has index 2 in G, then H is normal in G. A_3 has index 2 in S_3 because by Corollary 1 of Lagrange's Theorem, the index of H in G = |G|/|H|. In this case, G = S_3 and H = A_3. We know that |S_3| = 3!, and by Theorem 5.7 we know that |A_3| = 3!/2. Therefore, the index of A_3 in S_3 = 3!/(3!/2) = 2, and A_3 is normal in S_3. | ||
Revision as of 17:07, 1 October 2008
Prove that A_3 inside S_3 is normal.
To prove this use the definition that if a group is normal then as a collection left cosets equal right cosets.
I used S_3 to equal a symmetric triangle with {1,l,r,a,b,c} A_3 the group of even permutations in S_3 which are {1,l,r}
Right Cosets Left Cosets
A_3 * 1 1 * A_3
--Robertsr 19:58, 1 October 2008 (UTC)
I used what we proved in Ch 9 problem 7 to prove this. In that problem, we proved that if H has index 2 in G, then H is normal in G. A_3 has index 2 in S_3 because by Corollary 1 of Lagrange's Theorem, the index of H in G = |G|/|H|. In this case, G = S_3 and H = A_3. We know that |S_3| = 3!, and by Theorem 5.7 we know that |A_3| = 3!/2. Therefore, the index of A_3 in S_3 = 3!/(3!/2) = 2, and A_3 is normal in S_3.
