- In mathematics and physics, a small-world network is a type of mathematical graph in which most nodes are not neighbors of one another, but most nodes can be reached from every other by a small number of hops or steps. A small world network, where nodes represent people and edges connect people that know each other, captures the small world phenomenon of strangers being linked by a mutual acquaintance.
- In mathematical logic, Gödel's incompleteness theorems, proved by Kurt Gödel in 1931, are two theorems stating inherent limitations of all but the most trivial formal systems for arithmetic of mathematical interest. The theorems are of considerable importance to the philosophy of mathematics. They are widely regarded as showing that Hilbert's program to find a complete and consistent set of axioms for all of mathematics is impossible, thus giving a negative answer to Hilbert's second problem.
- We've all heard of 'six degrees of separation', the idea that everyone in the world can be connected in just a few steps. But what if those steps don't just relate to people but also to viruses, neurons, proteins and even to fashion trends? What if this 'six degrees of separation' allowed us an insight into something at the core of Nature?

*STOC '06: Proceedings of the thirty-eighth annual ACM symposium on Theory of computing,**page 71--78.**New York, NY, USA,**ACM,*(*2006*)*Decision Support Systems**15(4):251--266*(*December 1995*)