Richard Dedekind
Dedekind was born in Brunswick, the birthplace of Gauss, and received his degree under Gauss at Göttingen. At Göttingen he also studied with Dirichlet and was a close friend of Riemann; he was eventually to edit the works of all three. Dedekind's construction of the real numbers using `Dedekind cuts' was part of the effort of Dedekind, Cantor, and Weierstrass, and others to bring a rigor to analysis; earlier attempts such as those by Cauchy and Bolzano were hindered by a lack of understanding of irrational numbers. Dedekind summarized this work in his famous book Was sind und was sollen die Zahlen? (What are the numbers and what do they mean?). These contributions, called part of the aritmetization of analysis, illustrate Dedekind's arithmetic and algebraic viewpoint; as a professor he was probably the first to give lectures on Galois theory. In algebraic number theory Dedekind introduced his theory of ideals to restore unique factorization; today integral domains in which every ideal is a unique product of prime ideals are called Dedekind domains. The stamp below presents an ideal written as a product of prime ideals.
This stamp is one of my favorites; my Ph.D. thesis at the University of Wisconsin in 1970 was concerned with rings that were noncommutative analogs of Dededind domains.

Richard Dedekind (18311916)
German Democratic Republic Repubic (1981), No. 2181