Take the 2-minute tour ×
MathOverflow is a question and answer site for professional mathematicians. It's 100% free, no registration required.

Does there exist a (meromorphic) elementary function $f(z)$ that is zero at all the positive integers $z = 1, 2, 3, \ldots$ and only at those points?

Edit: an elementary function can be written as a finite composition of constants, rational functions, exponentials and logarithms.

Obviously a function with those zeros can be constructed using the gamma function or a Weierstrass product, but the question is whether there is an elementary function.

share|improve this question
    
sin(pi*x) maybe? –  jef May 8 '10 at 18:45
    
oh sorry, misread the question. –  jef May 8 '10 at 18:48
    
You should provide a definition of elementary. –  Qiaochu Yuan May 8 '10 at 18:51
add comment

2 Answers 2

$1/\Gamma(1-z)$.

share|improve this answer
    
The gamma function is not elementary. –  Fredrik Johansson May 8 '10 at 18:46
1  
This is, however, the "most elementary" function with this property, for some reasonable notions of elementary (at least, assuming you want simple zeroes). For instance, any other function with the same zeroes has larger asymptotic growth; this is a version of the Hadamard Factorization Theorem. More precisely, if $f(z)$ has simple zeroes at the positive integers, then there is a polynomial $g(z)$ so that $f(z) = e^{g(z)}/\Gamma(1-z)$. So if you don't think the gamma function is elementary, the answer to your original question is "no". –  Dylan Thurston May 9 '10 at 1:50
    
Sorry, I just reread the question and saw "meromorphic", which changes things. –  Dylan Thurston May 9 '10 at 1:51
add comment

I don´t completely understand what you mean by elementary, but you can look at http://en.wikipedia.org/wiki/Weierstrass_factorization_theorem. Sorry if the question had not to do with this.

share|improve this answer
    
To spell this out further: all the OP has left unspecified in the question is the degrees of vanishing. So to rephrase: is there any sequence d_1,d_2,... of strictly positive integers s.t. the function Weierstrass gives one is elementary? The OP has declared that having all d_i = 1 doesn't pass muster. –  Allen Knutson May 9 '10 at 1:29
    
@Allen: We're also allowed to have poles. –  Dylan Thurston May 9 '10 at 14:42
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.