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: <math> \begin{pmatrix} 3 & -4 \\ -2 & 3 \end{pmatrix} </math>
is an automorphism of the form <math>f = x^2 - 2y^2</math>. The automorphisms of a form are a [[subgroup]] of <math>\mathrm{SL}_2(\mathbb{Z})</math>. When ''f'' is definite, the group is finite, and when ''f'' is indefinite, it is infinite and [[cyclic group|cyclic]].
==Representation==
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