Polynomials having a common root with their derivatives - MathOverflow

Polynomials having a common root with their derivatives

2010-06-11T18:52:37Z

Here is a question someone asked me a couple of years ago. I remember having spent a day or two thinking about it but did not manage to solve it. This may be an open problem, in which case I'd be interested to know the status of it.

Let $f$ be a one variable complex polynomial. Supposing $f$ has a common root with every $f^{(i)},i=1,\ldots,\deg f-1$, does it follow that $f$ is a power of a degree 1 polynomial?

upd: as pointed out by Pedro, this is indeed a conjecture (which makes me feel less badly about not being able to do it). But still the question about its status remains.

Answer by Pedro Teixeira for Polynomials having a common root with their derivatives

2010-06-11T19:43:49Z

That is known as the Casas-Alvero conjecture. Check this out, for instance:

http://front.math.ucdavis.edu/0605.5090

Not sure of its current status, though.

Answer by Andrey Rekalo for Polynomials having a common root with their derivatives

2010-06-11T20:27:25Z

The strongest result in this direction that I've heard of is Sudbery's theorem (which was originally conjectured by Popoviciu and Erdös).

Theorem. Let $P(z)$ be a polynomial of degree $n\geq 2$ and let $\Pi(z)=\prod\limits_{k=0}^{n-1}P^{(k)}(z)$ where $P^{(k)}$ is the $k$th derivative of $P$. Then either $\Pi(z)$ has exactly one distinct root or $\Pi(z)$ has at least $n+1$ distinct roots.

See the original paper by Sudbery.