
From adele.com.
There has been a lot of news lately about Adele, which I found very exciting because adeles are important in mathematics. In number theory, it is helpful to complete the rational numbers (in the sense of topology), since then you can do analysis. The standard completion yields the real numbers. However there is extra arithmetical information to be found in the $p$-adic completions. But how do you know which $p$ to pick? Why not pick all of them at once! The adeles give you a way to do that (and to throw in the real numbers, too.) They play a central role in class field theory. Also, in the study of algebraic groups, introducing the adeles leads to adelic algebraic groups. (The previous link gives some history on the naming of the adeles.) A few key people who used adeles, as well as the ideles, early on are Claude Chevalley, Armand Borel, André Weil, John Tate, and Kenkichi Iwasawa. (They used different names for the ring until the world settled on Weil’s name for it.) The next time you hear “Hello”, followed by a pause, I hope you will join me in saying, “It’s $\mathbb{R}\times \prod’_{p}\mathbb{Q}_p$”.
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