# Nash Equilibrium Existence

Topics: Nash equilibrium, Game theory, Continuous function Pages: 5 (1685 words) Published: January 18, 2011
FIXED POINTS AS NASH EQUILIBRIA
´ JUAN PABLO TORRES-MARTINEZ Received 27 March 2006; Revised 19 September 2006; Accepted 1 October 2006

The existence of ﬁxed points for single or multivalued mappings is obtained as a corollary of Nash equilibrium existence in ﬁnitely many players games. ı Copyright © 2006 Juan Pablo Torres-Mart´nez. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Introduction In game theory, the existence of equilibrium was uniformly obtained by the application of a ﬁxed point theorem. In fact, Nash [3, 4] shows the existence of equilibria for noncooperative static games as a direct consequence of Brouwer [1] or Kakutani [2] theorems. More precisely, under some regularity conditions, given a game, there always exists a correspondence whose ﬁxed points coincide with the equilibrium points of the game. However, it is natural to ask whether ﬁxed points arguments are in fact necessary tools to guarantee the Nash equilibrium existence. (In this direction, Zhao [5] shows the equivalence between Nash equilibrium existence theorem and Kakutani (or Brouwer) ﬁxed point theorem in an indirect way. However, as he points out, a constructive proof is preferable. In fact, any pair of logical sentences A and B that are true will be equivalent (in an indirect way). For instance, to show that A implies B it is suﬃcient to repeat the proof of B.) For this reason, we study conditions to assure that ﬁxed points of a continuous function, or of a closed-graph correspondence, can be attained as Nash equilibria of a noncooperative game. 2. Deﬁnitions Let Y ⊂ Rn be a convex set. A function v : Y → R is quasiconcave if, for each λ ∈ (0,1), we have v(λy1 + (1 − λ)y2 ) ≥ min{v(y1 ),v(y2 )}, for all (y1 , y2 ) ∈ Y × Y . Moreover, if for each pair (y1 , y2 ) ∈ Y × Y such that y1 = y2 the inequality above is strict, independently of the value of λ ∈ (0,1), we say that v is strictly quasiconcave. Hindawi Publishing Corporation Fixed Point Theory and Applications Volume 2006, Article ID 36135, Pages 1–4 DOI 10.1155/FPTA/2006/36135

2

Fixed points as Nash equilibria

A mapping f : X ⊂ Rm → X has a ﬁxed point if there is x ∈ X such that f (x) = x. A vector x ∈ X is a ﬁxed point of a correspondence Φ : X X if x ∈ Φ(x). Given a game = {I,Si ,vi }, in which each player i ∈ I = {1,2,...,n} is characterized by a set of strategies Si ⊂ Rni , and by an objective function vi : n=1 S j → R, a Nash equij librium is a vector s = (s1 ,s2 ,...,sn ) ∈ Πn=1 Si , such that vi (s) ≥ vi (si ,s−i ), for all si ∈ Si , for i all i ∈ I, where s−i = (s1 ,...,si−1 ,si+1 ,... ,sn ). Finally, let = {S : ∃n ∈ N, S ⊂ Rn is nonempty, convex, and compact}. 3. Main Results Consider the following statements. [Nash-1]. Given = {I,Si ,vi }, suppose that each set Si ∈ and that objective functions are continuous in its domains and strictly quasiconcave in its own strategy. Then there is a Nash equilibrium for . [Nash-2]. Given = {I,Si ,vi }, suppose that each set Si ∈ and that objective functions are continuous in its domains and quasiconcave in its own strategy. Then there is a Nash equilibrium for . [Brouwer]. Given X ∈ , every continuous function f : X → X has a ﬁxed point.

[Kakutani∗ ]. Given X ∈ , every closed-graph correspondence Φ : X X, with Φ(x) ∈ for all x ∈ X, has a ﬁxed point, provided that Φ(x) = m 1 π m (Φ(x)) for each x ∈ X ⊂ j j= m . (For each j ∈ {1,...,m}, the projections π m : Rm → R are deﬁned by π m (x) = x , R j j j where x = (x1 ,...,xm ) ∈ Rm .) (The last property, Φ(x) = m 1 π m (Φ(x)), is not necessary j j= to assure the existence of a ﬁxed point, provided that the other assumptions hold. However, when objective functions are quasiconcave, [Kakutani∗ ] is suﬃcient to assure the existence of a Nash equilibrium.) Our results are [Nash-1] → [Brouwer]. Proof. Given a...