When are There Infinitely Many Irreducible Elements in a Principal Ideal   Domain?
  It has been a well-known fact since Euclid's time that there exist infinitely many rational primes. Two natural questions arise: In which other rings, sufficiently similar to the integers, are there infinitely many irreducible elements? Is there a unifying algebraic concept that characterizes such rings? The purpose of this note is to place the fact concerning the infinity of primes into a more general context, one that also includes the interesting case of the factorial domains of algebraic integers in a number field. We show that, if $A$ is a P.I.D., then $A$ contains infinitely many (pairwise nonassociate) irreducible elements if and only if every maximal ideal of $A[x]$ has the same (maximal) height. 
Fabrizio Zanello