# Continous analogue of Hirsch conjecture

\begin{conjecture} The order of the largest total curvature of the primal central path over all polytopes defined by $n$ inequalities in dimension $d$ is $n$. \end{conjecture}

Let $\lambda^c(P)$ denote the total curvature of the central path corresponding to the linear optimization problem $\min \{ c^Tx : x\in P\}$. The quantity $\lambda^c(P)$ can be regarded as the continuous analogue of the edge-length of the shortest path between a pair of vertices. Considering the largest $\lambda^c(P)$ over all possible $c$ we obtain the quantity $\lambda(P)$, referred to as the curvature of a polytope. Following the analogy with the diameter, let $\Lambda(d,n)$ be the largest total curvature $\lambda(P)$ of the primal central path over all polytopes $P$ defined by $n$ inequalities in dimension $d$.

Holt and Klee~\cite{HK} showed that, for $n> d\geq 13$, the conjecture of Hirsch is tight. We have the following continuous analogue of the result of Holt and Klee:

\cite{DTZa} $\liminf_{n\rightarrow\infty}\frac{\Lambda(d,n)}{n}\geq \pi$, that is, $\Lambda(d,n)$ is bounded below by a constant times $n$.

The special case of $n=2d$ of the conjecture of Hirsch is known as the $d$-step conjecture, and it has been shown by Klee and Walkup~\cite{KW} that the $d$-step conjecture is equivalent to the Hirsch conjecture. We have the following continuous analogue of the result of Klee and Walkup:

\cite{DTZb} If the order of the curvature is less than the dimension $d$ for all polytope defined by $2d$ inequalities and for all $d$, then the order of the curvature is less that the number of inequalities for all polytopes; that is, if $\Lambda(d,2d)=\mathcal{O}(d)$ for all $d$, then $\Lambda(d,n)=\mathcal{O}(n)$.

## Bibliography

[DMS] J.-P. Dedieu, G. Malajovich and M. Shub: On the curvature of the central path of linear programming % theory. Foundations of Computational Mathematics. 5 (2005) 145--171.

*[DTZa] A. Deza, T. Terlaky and Y. Zinchenko: Polytopes and arrangements : diameter and curvature. Operations Research Letters (to appear).

[DTZb] A. Deza, T. Terlaky and Y. Zinchenko: The continuous d-step conjecture for polytopes. AdvOL-Report 2007/16, McMaster University (2007).

[HK] F. Holt and V. Klee: Many polytopes meeting the conjectured Hirsch bound. Discrete and Computational Geometry 20 (1998) 1--17.

[KW] V. Klee and D. Walkup: The $d$-step conjecture for polyhedra of dimension $d<6$. Acta Mathematica 133 (1967) 53--78.

* indicates original appearance(s) of problem.