Entire function interpolation with control over multiplicities/derivatives - MathOverflow

Entire function interpolation with control over multiplicities/derivatives

2010-05-19T08:08:27Z

Let's say I have a multiset of complex numbers $\lbrace a_1,\cdots,a_n\rbrace$ (so some of the elements may be repeated) and I would like to construct an entire function $p(z)$ with those numbers as zeroes. However, I also have a multiset of complex numbers $B = \lbrace b_1,\cdots,b_n \rbrace$ such that I wish $p(b_i) = 1$ - p is only 1 on the $b_i$'s.

It seems like trying to use Lagrange's polynomial interpolation formula gives you a polynomial with too high a degree (greater than $n$ and less than or equal to $2n$), and then there's the possibility that $p^{-1}(1) \nsubseteq B$.

I've been thinking about doing the following:

Let $g(z) = (x-a_1) \cdots (x - a_n)$, and then via Weierstrass construct an entire function $h(z)$ such that $e^{h(b_i)} = 1/g(b_i)$. Then it seems like the entire function $e^{h(z)}g(z)$ is getting somewhat closer to what I want - but then again I don't know if there are any other $\alpha$'s such that $e^{h(\alpha)}g(\alpha) = 1$ where $\alpha \notin B$.

The problem of polynomial interpolation and fitting seems very well studied; however, I can't seem to find a reference for this particular puzzle.

Thanks in advance!

Answer by Robin Chapman for Entire function interpolation with control over multiplicities/derivatives

2010-05-19T08:38:09Z

If I read you right, you want an entire function that takes the values $0$ and $1$ at only finitely many (specified) points. This implies that the function must be a polynomial, by Picard's great theorem, since there will be deleted neighbourhoods of infinity where the function misses two values.

Answer by Roland Bacher for Entire function interpolation with control over multiplicities/derivatives

2010-05-19T11:25:21Z

Some very nice instances of your problem (but of course not all) are solved by so-called Shabbat polynomials, ie. by polyomials such that $p^{-1}[0,1]$ is a tree and such that ${0,1,\infty}$ are the only critical values. Every planar tree can be realized by an (essentially unique up to affine transformation) Shabbat polynomial. You have thus a polynomial solution if your points $a_i$ and $b_i$ form a bipartition of the vertices of such a "Shabbat-tree".

Let me add that Shabbat polynomials are the simplest instances of "dessins d'enfants" defined by Grothendieck in the hope of understanding the absolute Galois group. (Suitably normalized Shabbat polynomials have algebraic coefficients and the action of the absolute Galois group preserves them and acts thus on the corresponding trees by permuting them.)

Answer by Alexandre Eremenko for Entire function interpolation with control over multiplicities/derivatives

2012-08-12T12:34:15Z

In your statement, you do not say explicitly, whether $p$ is aloowed to have other zeros, except those in the set $A$.

If you want to construct an entire function with zeros and ones exactly prescribed, this is clearly impossible when your sets $A$ and $B$ are both finite. For the reason explained by Robin Chapman.

If you want ones to be exactly prescribed, and function having zeros on the set $A$, and perhaps other zeros, then this is possible: take $p(z)=1+(z-b_1)...(z-b_n)\exp g(z)$ and use interpolation for $g$.