Circle packing in the hyperbolic plane.
Classification of hyperbolic dodecahedron tilings of maximally face-transitive motion groups. (Klassifikation der hyperbolischen Dodekaederpflasterungen von maximalen flächentransitiven Bewegungsgruppen.)
Cobordism of automorphisms of surfaces
Comparision of the length of infinite geodesics in manifolds with nonpositive curvature.
Correction to "On a functional equation arising from hyperbolic geometry", Volume 21 (1980), 113-116.