1. N. I. Feldman and Yu. V. Nesterenko, “Transcendental Numbers,” in Encyclopaedia of Mathematical Sciences, Vol. 44: Number Theory IV (Springer, 1998).
2. F. Rudio, Archimedes, Huygens, Lambert, Legendre. Vier Abhandlungen über die Kreismessung (Teubner, Leipzig, 1892).