You are currently browsing the tag archive for the ‘Euclid’ tag.

This post serves to resolve some questions posed at the end of a previous article, Unit-Prime Dichotomy for Complex Subrings, in which we began to look at the tension between the relative sizes of the units group and the spectrum of a complex subring (with unity). As a disclaimer, I assume from this point familiarity with the topics presented therein. To begin, we recall a Theorem from the prequel (Theorem 4):

**Theorem: ***Let be a subring (with unity), and suppose that has finite spectrum. Then is dense in , the topological closure of the fraction field of .*

In particular — assuming a finite spectrum — it follows that the rank of (considered as an abelian group) is bounded below by two. Unfortunately, this is as far as our previous methods will take us, even when (see the Exercises).

The primary objective of this post is an extension to this result. Specifically, we would like to capture *in a purely algebraic way* (e.g. without mention of the topology on ) the fact that becomes quite large in certain rings with finite spectrum. After the fold we’ll accomplish exactly that, with the following Theorem and its generalizations to a wide class of commutative rings:

**Theorem 1: ***Suppose that is a subring (with unity). If has finite spectrum, then has infinite rank as an abelian group.*

Throughout, we take as a complex subring (with unity). In this article, we’ll be interested in natural analogues of Euclid’s proof of the infinitude of the primes (i.e. the case . In particular, we’ll show that Euclid’s proof (and generalizations to this method) can be recast as a relationship between the size of the unit group of and , the number of prime ideals in. *(Here, denotes the spectrum of .)*

For the sake of explicit analogy, we include a (needlessly abstracted) version of Euclid’s result now:

**Theorem 1 (Euclid): ** *The integers have infinite spectrum.*

*Proof: *If not, let exhaust the list of prime ideals. As the integers form a PID, (in fact, they form a Euclidean domain), we may associate to each given ideal a prime generator such that . Let ; as is not a unit (replacing with if necessary), it admits a prime factor which equals some . But then divides both and , whence as well. This is a contradiction, and our result follows.

It is worth remarking that the “PID-ness” of the integers is not needed in the derivation of Theorem 1. Indeed, if were not principal, we may take instead to be *any* non-zero element of . Thus, we see that the PID-ness of is *not* the crucial property of the integers (viewed as a subfield of ) upon which our proof stands. Rather — as we’ll see after the fold — Euclid’s proof frames the infinitude of primes as a consequence of the *finiteness of the units group!*