Ellipses and harmonic Möbius transformations. Özgür, Nihal Yilmaz — 2010 Analele Ştiinţifice ale Universităţii “Ovidius" Constanţa. Seria: Matematică
On the principal congruence subgroups of the Hecke-group H ( 5 ) . Yılmaz Özgür, Nihal; Cangül, İ. Naci — 2004 Beiträge zur Algebra und Geometrie