1. [1] P.J. Nahin, Dr. Euler’s fabulous formula, Cures Many Mathematical Ills, Princeton University Press, 2006.
2. [2] R. Wilson, Euler’s Pioneering Equation: The Most Beautiful Theorem in Mathematics, Oxford University Press, 2018.
3. [3] R. Penrose, Conformal approach to infinity, in Relativity, Groups and Topology, Gordon and Breach, New York, 1964.
4. [4] R. Penrose, Topological QFT and twistors: holomorphic linking, Twistors Newsletter, vol. 27, 1-4, 1988.
5. [5] R. Penrose, Newton, quantum theory and reality, in 300 years of Gravity (ed. S.W. Hawking and W. Israel), pp. 17-49, Cambridge University Press, Cambridge, 1987.