Geometric Convergence of Rational Approximations to e-z in Infinite Sectors. E.B. Saff, R.S. Varga, W.C. Ni (1976) Numerische Mathematik
Geometrie Convergence to e-z by Rational Functions with Real Poles. A. Schönhage, E.B. Saff, R.S. Varga (1975/1976) Numerische Mathematik