In geometric topology, the side-approximation theorem was proved by Bing (1963). It implies that a 2-sphere in R3 can be approximated by polyhedral 2-spheres.
Bing, R. H. (1963), "Approximating surfaces from the side", Annals of Mathematics, Second Series, 77: 145–192, doi:10.2307/1970203, ISSN 0003-486X, JSTOR 1970203, MR 0150744