In this note, we give self-contained proofs of the following three theorems: Theorem A (Auslander-Buchsbaum formula) The depth of a Noetherian local ring A is equal to the sum of the depth and the projective dimension of a nitely generated A-module M with nite projective dimension. Theorem B (Serre theorem) A Noetherian local ring is regular if and only if its global dimension is nite. Theorem C A regular local ring is a unique factorization domain.