This number is used in mathematics to show that mathematics is a strongly interlinked science. Build a graph where each mathematician is a vertex and a connection edge between two vertices indicates that these two mathematics have published a joint paper. Then, the Erdös number of a given mathematician is the minimal number of edges (=joint papers) you need to reach Erdös from that person. Erdös was a famous mathematician with about 1000 publications and around 100 different coauthors (the ones with Erdös number 1)
0. Paul Erdös
Paul Erdös & David Preiss: Decomposition of spheres in Hilbert spaces. Comment. Math. Univ. Carolinae 17 (1976), no. 4, 791--795.
1. David Preiss
David Preiss & Vladimír Sverák: Derivatives of type $1$. Real Anal. Exchange 12 (1986/87) 354--360.
2. Vladimír Sverák
Vladimír Sverák & Tomás Roubíček: Nonexistence of solutions in nonconvex multidimensional variational problems. J. Convex Anal. 7 (2000) 427--435.
3. Tomáš Roubíček
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