I recall the following question from Ulam's book "Unsolved math problems": show that the ring of Dirichlet series with integer coefficients is a factorial ring. I believe that soon after Ulam wrote his book, this problem was solved; essentially, this ring is isomorphic to the ring of formal power series in infinitely many variables (one for each prime) with integer coefficients, and once it is reformulated this way it looks much less mysterious. However, my actual question is not about the proof,  I was always curious if a result like that (in its original formulation) can be really applied or interpreted from the Dirichlet series point of view. Has anyone heard/thought of any reasonable numbertheoretic interpretation of this fact, or does the interpretation as a power series ring kill any hope of that sort?
2 Answers
First, I want to nail down a reference to the solution to the problem alluded to above: the Dirichlet ring of functions f: Z^{+} > Z with pointwise addition and convolution product is a UFD. This was proved by L. Durst in his 1961 master's thesis at Rice University and independently by Cashwell and Everett. References:
Deckard, Don; Durst, L. K. Unique factorization in power series rings and semigroups. Pacific J. Math. 16 1966 239242.
Cashwell, E. D.; Everett, C. J. Formal power series. Pacific J. Math. 13 1963 4564.
Note that the latter is the second paper of Cashwell and Everett on the subject of factorization in Dirichlet rings. They also had a 1959 paper:
Cashwell, E. D.; Everett, C. J. The ring of numbertheoretic functions. Pacific J. Math. 9 1959 975985.
Ulam makes reference to the 1959 paper in his statement of the problem in the first (1960) edition of his book. In the 1964 paperback edition of his book, he announces the problem as solved by 1961 papers of Buchsbaum and Samuel.
I want to make the remark that since the original 1959 paper of Cashwell and Everett certainly makes the connection to formal power series rings, there is more to the solution of the problem than this. (Indeed, the 1961 result of Samuel, that if R is a regular UFD, then R[[t]] is again a UFD, seems to be the key.)
There is a 2001 arxiv preprint of Durst which claims a more elementary proof of the result:
http://arxiv.org/PS_cache/math/pdf/0105/0105219v1.pdf
As to whether the result has numbertheoretic significance: not as far as I know. I should say that I first learned of the CashwellEverett theorem by reading an analytic number theory text  Tenenbaum, Introduction to Analytic and Probabilistic Number Theory, p. 26  but the author seems to mention it just for culture.
Finally, remember that this is a result about factorization in the ring of formal Dirichlet series, an inherently algebraic beast. So perhaps the following result is more relevant:
Bayart, Frédéric; Mouze, Augustin Factorialité de l'anneau des séries de Dirichlet analytiques. (French) [The ring of analytic Dirichlet series is factorial] C. R. Math. Acad. Sci. Paris 336 (2003), no. 3, 213218

$\begingroup$ Last time I saw the Ulam's book (a Russian translation of the 1st edition) was almost ten years ago, so I completely forgot the names he referred to, but now that you mention Cashwell and Everett, I recall it instantly that they were mentioned there. Your last link is indeed much more exciting  and indeed more likely to be relevant. It'd be quite interesting to know if there is some numbertheoretic meaning of that. $\endgroup$ Nov 14, 2009 at 19:04
I believe so. Dirichlet series are a special case of a construction called the reduced incidence algebra of a poset, and the poset of integers under division is precisely a product of chains, one for each prime. Finally, the reduced incidence algebra of a chain is just the ring of power series in one variable.

$\begingroup$ So why would expect incidence algebras to be UFDs then??? $\endgroup$ Nov 14, 2009 at 16:44