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 | | The centre of the group has order 2, and is generated by x^2 = -I. The inner automorphism group is the group modulo its centre, which is PSL(2,Z), and is isomorphic to the free product C_2 * C_3. |
 | | The full automorphism group is at least twice as big as that, because it contains PGL(2,Z), which is the image of integral matrices with determinant 1 or -1. |
 | | Each inner automorphism of G induces the trivial automorphism of G/[G,G], so the inner automorphisms of G lie in ker(phi). |
| www.math.niu.edu /~rusin/known-math/01_incoming/SL2Z (938 words) |
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