Let {a_{n}} be a sequence of complex numbers indexed by the positive integers. Does there always exist an analytic function f such that f(n) = a_{n} for n=1,2,...? If not, are there any simple necessary or sufficient conditions for the existence of such f? This analytic function should be defined on some connected domain in the complex plane containing the positive integers.

To make this concrete, consider Ackermann's function, which is defined recursively: first define the sequence of functions A_{k}, k=1,2,..., as

A_{1}(n) = 2n,

A_{k}(1) = 2,
A_{k}(n) = A_{k-1}(A_{k}(n-1)),

and then define Ackermann's function as the diagonal A(n) = A_{n}(n) for n >= 1. Does there exist an analytic function f such that f(n) = A(n) for n = 1,2,...?

Actually, the individual functions A_{k} are interesting as well. A_{1}(n) = 2n, as given above; A_{2}(n) = 2^{n}, and A_{3}(n) = 2^(2^(...2^2)) (with n twos in the expression). Obviously, A_{1} and A_{2} have analytic extensions. According to Wikipedia (which uses a slightly different definition and notation), analytic extensions of A_{3} or A_{k} for any other k aren't known, but from the language, it isn't clear whether the existence of an extension is itself in question, or whether one simply hasn't been found yet. Also, it doesn't say anything about the diagonal A(n) (unless I missed it).

There are many other obvious sequences that don't seem to have obvious analytic extensions, like the prime-counting function (just to name one!). As far as my knowledge is concerned, this seems more like the rule than the exception. My knowledge here is admittedly very limited, though, so anything at all that you can share will probably teach me something.