Sanov subgroup in SL(2,Z)

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This article is about a particular subgroup in a group, up to equivalence of subgroups (i.e., an isomorphism of groups that induces the corresponding isomorphism of subgroups). The subgroup is (up to isomorphism) free group:F2 and the group is (up to isomorphism) special linear group:SL(2,Z) (see subgroup structure of special linear group:SL(2,Z)).
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This is the subgroup of special linear group:SL(2,Z) generated by the matrices:

\begin{pmatrix} 1 & 1 \\ 0 & 1 \\\end{pmatrix}, \qquad \begin{pmatrix} 1 & 0 \\ 1 & 1 \\\end{pmatrix}

It is a free group of rank two with the above two elements as a freely generating set for it. Further information: Sanov subgroup in SL(2,Z) is free of rank two

Arithmetic functions

The subgroup has index 12 in the whole group. In fact, any finite index free subgroup of rank two in the special linear group of degree two must have index 12.

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