2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/173675This is an easy-reading which describes few geometric invariants which can be obtained from the group SL(2,R) within the Erlangen program of F.Klein.AMS-LaTeX, 9 pages, 8 figures; v2: small improvements; v3: small corrections, more links to Wikipedia added; v4: minor corrections, two references addedGeneral MathematicsGroup TheoryHistory and Overview00-01, 00A05, 51-01, 51-02text