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  [[20XCLLT93[120X]  [16XChan,  S.-P.,  Lang,  M.-L.,  Lim,  C.-H.  and Tan, S. P.[116X, [17XSpecial
  polygons  for  subgroups of the modular group and applications[117X, [18XInternat. J.
  Math.[118X, [19X4[119X, 1 (1993), 11--34.
  
  [[20XKul91[120X]  [16XKulkarni, R. S.[116X, [17XAn arithmetic-geometric method in the study of the
  subgroups of the modular group[117X, [18XAmer. J. Math.[118X, [19X113[119X, 6 (1991), 1053--1133.
  
  [[20XLLT95a[120X]  [16XLang,  M.-L.,  Lim,  C.-H.  and  Tan,  S.  P.[116X,  [17XAn  algorithm  for
  determining  if  a  subgroup  of  the modular group is congruence[117X, [18XJ. London
  Math. Soc. (2)[118X, [19X51[119X, 3 (1995), 491--502.
  
  [[20XLLT95b[120X]  [16XLang, M.-L., Lim, C.-H. and Tan, S. P.[116X, [17XIndependent generators for
  congruence subgroups of Hecke groups[117X, [18XMath. Z.[118X, [19X220[119X, 4 (1995), 569--594.
  
  
  
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