The vast majority of positive results about computational problems are constructive proofs, i.e., a computational problem is proved to be solvable by showing an algorithm that solves it; a computational problem is shown to be in P (complexity) by showing an algorithm that solves it in time that is polynomial in the size of the input; etc. However, there are several non-constructive results, where an algorithm is proved to exist without showing the algorithm itself. Several techniques are used to provide such existence proofs.