Computing the integral closure of an affine semigroup. Bruns, Winfried; Koch, Robert — 2001 Zeszyty Naukowe Uniwersytetu Jagiellońskiego. Universitatis Iagellonicae Acta Mathematica
Non-Euclidean Points in Sphere-like Spaces. Robert J. Koch; Haskell Cohen — 1968 Mathematische Zeitschrift