Asymptotic behaviour of fixed-order error constants of modified quadrature formulae for Cauchy principal value integrals. Diethelm, Kai; Köhler, Peter — 2000 Journal of Inequalities and Applications [electronic only]
On the error of compound quadrature formulas for r-convex functions. Peter Köhler — 1990 Aequationes mathematicae
Brouwerian semilattices: The lattice of total subalgebras Peter Köhler — 1982 Banach Center Publications