MathSciNet bibliographic data MR702298 30C20 (03C60 20G20 30C25) Rubel, Lee A. Conformal inequivalence of annuli and the first-order theory of subgroups of ${\rm PSL}(2,\,{\bf R})$${\rm PSL}(2,\,{\bf R})$. Proc. Amer. Math. Soc. 88 (1983), no. 4, 679–683. Article

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