1. Tegmark, M. in Science and Ultimate Reality: From Quantum to Cosmos (eds Barrow, J. D., Davies, P. C. W. & Harper, C. L. Jr) Ch. 21 (Cambridge Univ. Press, Cambridge, 2004).
2. Vilenkin, A. Many Worlds in One: The Search for Other Universes (Hill & Wang, New York, NY, 2007).
3. Hilbert, D. in David Hilbert’s Lectures on the Foundations of Arithmetics and Logic 1917–1933 (eds Ewald, W. & Sieg, W.) 730 (Springer, Heidelberg, 2013).
4. Nicolai, H. CERN Courier
https://cerncourier.com/gravitys-quantum-side/
(2017).
5. Hawking, S. W. & Penrose, R. Proc. R. Soc. Lond. A 314, 529–548 (1970).