One of the major contributions of Paul Erdős is the development of the Probabilistic Method and its applications in Combinatorics, Graph Theory, Additive Number Theory and Combinatorial Geometry. This short paper describes some of the beautiful applications of the method, focusing on the long-term impact of the work, questions and results of Erdős. This is mostly a survey, but it contains a few novel results as well.
The Probabilistic MethodThe Probabilistic Method is one of the most significant contributions of Paul Erdős, and part of his greatness is the fact that applications of the probabilistic method and of random graphs have become so common that it is now possible to use those without explicitly mentioning him. The method is a powerful tool with numerous applications in Combinatorics, Graph theory, Additive Number Theory and Geometry and had an immense impact on the development of theoretical Computer Science as well. The results and tools are far too numerous to cover in a short survey, even if the focus is only on those influenced directly by the work and problems of Erdős, and thus this paper is mainly a selection of topics that illustrate the method, and is not meant to be a comprehensive treatment of the whole area. Several books that contain more material on the subject are [13], [18], [54], [61].It is convenient to classify the applications of probabilistic techniques in Discrete Mathematics into three groups. The first one deals with the study of random combinatorial objects, like random graphs or random matrices. The results here are essentially results in Probability Theory, although many of them are motivated by problems in Combinatorics. The second group consists of probabilistic constructions. These are applications of probabilistic arguments in order to prove the existence of combinatorial structures which satisfy a list of prescribed properties. Existence proofs of this type often supply extremal examples to various questions in Discrete Mathematics. The third group, which contains some of the most striking examples, focuses on the application of probabilistic reasoning in the proofs of deterministic statements whose formulation does not give any indication that randomness may be helpful in their study.