Joint seminar of the Optimization team of CERMICS (Ecole des Ponts

Paristech) and the Tropical team (INRIA Saclay & CMAP, Ecole

Polytechnique)

## Forthcoming sessions

## Kazuo Murota (The Institute of Statistical Mathematics, Tokyo)

### May 20th, 10:00, Online

## Multiple exchange property of gross substitutes valuations

with a proof by discrete convex analysis

Abstract:

Discrete convex analysis (DCA) offers a general framework of discrete optimization,

combining the ideas from matroid theory and convex analysis. It has found applications

in many different areas including operations research, mathematical economics, and game

theory. The interaction between DCA and mathematical economics was initiated by

Danilov, Koshevoy, and Murota (2001), and accelerated by the crucial observation of Fujishige

and Yang (2003) that M-natural-concavity is equivalent to the gross substitutes (GS) property

of Kelso and Crawford (1982).

In this talk we show how an old question in economics was settled with

the DCA machinery. More concretely, we explain how the equivalence of the gross substitutes

condition to the strong no complementarities (SNC) condition of Gul and Stacchetti

(1999) can be proved with the use of the Fenchel-type duality theorem and the conjugacy

theorem in DCA.

The SNC condition means the multiple exchange property of a set function

f, saying that, for two subsets X and Y and a subset I of X-Y, there exists a subset J

of Y-X such that f((X-I)+J) +f((Y-J)+I) is not smaller than f(X) + f(Y).

This talk is intended to offer a glimpse at a technical interaction

between DCA and economics.

No preliminary knowledge of DCA is required.

## Past sessions

## Guillaume Vigeral (Ceremade, Paris Dauphine)

### April 8th, 10:30, Online

## Structure of the sets of Nash equilibria of finite games; applications

to the complexity of some decision problems in game theory

Abstract:

The set of Nash equilibrium payoffs of a finite game is always non

empty, compact and semialgebraic. We show that, for 3 players or more,

the reverse also holds: given E a subset of R^N that is non empty,

compact and semialgebraic, one constructs a finite N player game such

that E is its set of equilibrium payoffs. Related results also holds

when one consider only games with integral payoffs, or when the focus is

on equilibria instead of equilibrium payoffs.

We apply this result to understand the complexity class of some natural

decision problems on finite games.