1. Rūmī: Poet and Mystic (1207–1273),1950
2. Space–time as a causal set;Bombelli;Phys. Rev. Lett.,1987
3. First steps with causal sets;Sorkin,1991
4. Space-time and causal sets;Sorkin,1991
5. Techniques of Differential Topology in Relativity;Penrose,1972