modular groupn.Mathematics the group of linear transformations z → az + b/ cz + d, where ad − bc = 1 (cf. modulusn. 2d(c), Möbius transformationn. at Möbiusn. 2).
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1898 Science 16 Sept. 362/2 The subgroups of the generalized modular group.
1990 Glasgow Math. Jrnl.32 317 Let Γ denote the modular group, consisting of the Möbius transformations z → az + b/ cz + d..ad − bc = 1.