Suppose A=[a] is a 2 x 2 matrix with integer entries. Let L be the
ij
2 2
module homomorphism from Z to Z given by matrix mulplication by A.
2 2
Suppose Z /L(Z ) is finite. Prove that its order is |a a -a a |.
11 22 12 21
想了好久還是沒有頭緒
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