Mean value problem

In mathematics, the mean value problem was posed by Stephen Smale in 1981. This problem is still open in full generality. The problem asks:

For a given complex polynomial $f$ of degree $d\ge 2$ and a complex number $z$, is there a critical point $c$ of $f$ (i.e. $f^{\prime }(c)=0$) such that

$\left|\frac{f(z)-f(c)}{z-c}\right|\le K|f^{\prime }(z)|\text{ for }K=1\text{?}$

It was proved for $K=4$. For a polynomial of degree $d$ the constant $K$ has to be at least $\frac{d-1}{d}$ from the example $f(z)=z^d-dz$, therefore no bound better than $K=1$ can exist.

Partial results

The conjecture is known to hold in special cases; for other cases, the bound on $K$ could be improved depending on the degree $d$, although no absolute bound $K<4$ is known that holds for all $d$.

In 1989, Tischler showed that the conjecture is true for the optimal bound $K=\frac{d-1}{d}$ if $f$ has only real roots, or if all roots of $f$ have the same norm.

In 2007, Conte et al. proved that $K\le 4\frac{d-1}{d+1}$, slightly improving on the bound $K\le 4$ for fixed $d$.

In the same year, Crane showed that $K<4-\frac{2.263}{\sqrt{d}}$ for $d\ge 8$.

Considering the reverse inequality, Dubinin and Sugawa have proven that (under the same conditions as above) there exists a critical point $\zeta$ such that $\left|\frac{f(z)-f(\zeta )}{z-\zeta }\right|\ge \frac{|f^{\prime }(z)|}{n{4}^n}$.

The problem of optimizing this lower bound is known as the dual mean value problem.

See also

• List of unsolved problems in mathematics