# Core in the theory of games

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The set of all non-dominated outcomes, that is, the set $C$ of outcomes such that a domination $s \succ _ {K} c$ cannot hold for any outcomes $s \in S$, $c \in C$ and coalition $K \in \mathfrak R _ {i}$. One defines in this respect:

1) The core. The set $c ( v)$ of imputations that are not dominated by any other imputation; the core coincides with the set of imputations satisfying $\sum _ {i \in S } x _ {i} \geq v ( S)$ for any coalition $S$. If $c ( v) \neq \emptyset$ and a von Neumann–Morgenstern solution (see Solution in game theory) exists, then $c ( v)$ is contained in any von Neumann–Morgenstern solution.

2) The kernel. The set $k ( v)$ of individually rational configurations $( x, \mathfrak B )$ (see Stability in game theory) such that the following inequality holds for any $i, j \in B \in \mathfrak B$:

$$\left ( \max _ {S \in \tau _ {ij} } e ( S, x) - \max _ {S \in \tau _ {ji} } e ( S, x) \right ) x _ {j} \leq 0,$$

where $e ( S, x) = v ( S) - \sum _ {k \in S } x _ {k}$ and $\tau _ {ij}$ is the set of coalitions containing the player $i$ and not containing the player $j$. The kernel $k ( v)$ is contained in an $M _ {1} ^ {i}$-bargaining set.

3) The nucleolus. The minimal imputation $n ( v)$ relative to the quasi-order $\prec _ \nu$ defined on the set of imputations by: $x \prec _ \nu y$ if and only if the vector $\theta ( x, v) = ( \theta _ {1} ( x, v) \dots \theta _ {n} ( x, v))$, where

$$\theta _ {i} ( x, v) = \max _ {\begin{array}{c} | \mathfrak U | = i \end{array} } \ \min _ {\begin{array}{c} S \in \mathfrak U \end{array} } e ( S, x) ,$$

lexicographically precedes $\theta ( y, v)$. The nucleolus $n ( v)$ exists and is unique for any game with a non-empty set of imputations. In a cooperative game the nucleolus is contained in the kernel.

#### References

 [1] N.N. Vorob'ev, "The present state of the theory of games" Russian Math. Surveys , 25 : 2 (1970) pp. 77–136 Uspekhi Mat. Nauk , 25 : 2 (1970) pp. 103–107

#### Comments

The Russian word ( "yadro" ) is the same for all three notions defined above, but these notions may be distinguished by prefixing with the corresponding English letter ( "c-yadro" for core, "k-yadro" for kernel and "n-yadro" for nucleolus). These three notions do not share many properties.

See [a1], [a7] for core, [a2] for kernel and [a3] for nucleolus. [a4], [a5] are general references. [a6] deals also with mathematical economics and the role of the concept of the core of a game in that setting.

#### References

 [a1] O.N. Bondareva, "Certain applications of the methods of linear programming to the theory of cooperative games" Probl. Kibernet , 10 (1963) pp. 119–139 (In Russian) [a2] M. Maschler, M. Davis, "The kernel of a cooperative game" Naval Res. Logist. Quart. , 12 (1965) pp. 223–259 [a3] D. Schmeidler, "The nucleolus of a characteristic function game" SIAM J. Appl. Math. , 17 (1969) pp. 1163–1170 [a4] G. Owen, "Game theory" , Acad. Press (1982) [a5] J. Szép, F. Forgó, "Introduction to the theory of games" , Reidel (1985) pp. 171; 199 [a6] J. Rosenmüller, "Cooperative games and markets" , North-Holland (1981) [a7] L.S. Shapley, "On balanced sets and cores" Naval Res. Logist. Quart. , 14 (1967) pp. 453–460
How to Cite This Entry:
Core in the theory of games. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Core_in_the_theory_of_games&oldid=52321
This article was adapted from an original article by A.I. Sobolev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article