KTH/SU Mathematics Colloquium

06-02-15

Mattias Jonsson, KTH

Isoperometric inequalities and the number of solutions to algebraic equations

The classical isoperometric inequality in the plane asserts that the area $S$ of a compact convex set and the length $L$ of its boundary satisfy $L^2 \ge 4\pi S$, with equality exactly in the case of a circle. This result, as well as a sharpening by Bonnesen, can be viewed as a special case of inequalities relating the mixed volumes of convex sets in Euclidean space, the most celebrated of which are the Alexandrov-Fenchel inequalities.

Algebraic Geometry is (or used to be) about solving polynomial equations. An important example is the Riemann-Roch problem: given a compact Riemann surface $\Sigma$, find the dimension of the vector space of rational functions on $\Sigma$ with predescribed poles. Generalizing this problem suitably to higher dimensions leads to the notion of volume of a divisor on a variety, the focus of much recent research activity.

It is well known that there is a dictionary between convex bodies and (toric) algebraic geometry. In particular, Teissier and Khovanskii showed that the Alexandrov-Fenchel inequalities can be derived from the Hodge Inequalities. The talk will be a non-technical survey of this circle of ideas, covering mostly classical material.