1. Abramowitz, M., Stegun, I.A.: Handbook of Mathematical Functions, with Formulas, Graphs, and Mathematical Tables. Dover Publications (1965)
2. Arndt, J., Haenel, C.: Pi
$$-$$
-
unleashed. Translated from the German in 1998 by C, p. 2001. Springer, Berlin, Lischka and Lischka, D., Second ed (2001)
3. Backhouse, N.: Pancake Functions and Approximations to
$$\pi $$
π
. Note 79.36, Math. Gazette 79, 371–374 (1995)
4. Balibrea, F., Linero, A.: Some new results and open problems on periodicity of difference equations. In: Iteration theory (ECIT 2004), 15–38, Grazer Math. Ber., 350, Karl-Franzens-Univ. Graz, Graz (2006)
5. Barnes, J.: Nice numbers. Birkhäuser/Springer, Cham (2016)