In graph theory, trees are combinatorial objects usually defined as connected graphs without cycles. In this thesis, the problem of counting different trees with a given number of vertices is presented. Four proofs of a celebrated theorem on the number of labeled trees by A. Cayley are given: by using a bijective construction due to H. Prüfer, by double counting of labeled rooted trees due to J. Pitman, by establishing a recursion on the number of labeled forests, and finally, by applying a relation between determinants and spanning trees due to H. Kirchhoff. In the final part, a recent result by A. Chin et al. is presented on the probability that a randomly picked tree in a complete graph is a spanning tree. In particular, the limit of this value as the number of vertices approaches infinity is observed. The result obtained is both surprising and beautiful.
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