Equilibrium

The word "equilibrium" refers, among other things, to concepts of game theory. The most well-known equilibrium in game theory is the Nash equilibrium.

Types of equilibria
According to https://en.wikipedia.org/w/index.php?title=List_of_types_of_equilibrium&oldid=1076576484#Game_theory
 * Correlated equilibrium, a solution concept in game theory that is more general than Nash equilibrium
 * Nash equilibrium, the basic solution concept in game theory
 * Quasi-perfect equilibrium, a refinement of Nash Equilibrium for extensive form games due to Eric van Damme
 * Sequential equilibrium, a refinement of Nash Equilibrium for games of incomplete information due to David M. Kreps and Robert Wilson
 * Perfect Bayesian equilibrium, a refinement of Nash equilibrium for games of incomplete information that is simpler to use than sequential equilibrium
 * Symmetric equilibrium, an equilibrium where all players use the same strategy
 * Trembling hand perfect equilibrium assumes that the players, through a "slip of the hand" or tremble, may choose unintended strategies
 * Proper equilibrium due to Roger B. Myerson, where costly trembles are made with smaller probabilities

According to some editors on electowiki: :

Nash equilibria are situations where each player has chosen a strategy and no single player could improve their situation by unilaterally changing strategy while all the other players keep their strategies.

Cabal equilibria or strong Nash equilibria are situations where each player has chosen a strategy and no subset of players could simultaneously change behaviour (while those outside that subset keep their strategies) in a way that no player in that subset is worse off and at least one in the subset is better off.

All cabal equilibria are Nash equilibria but not vice versa.

The term "cabal equilibrium" was devised by Peter de Blanc. It is difficult to find online by that name elsewhere.

Strong equilibrium
A strong Nash equilibrium is a set of ballots such that candidate X wins, and no set of voters can change their ballots such that a candidate Y whom all of them strictly prefer to X will win.

A slightly stronger and more restrictive concept is that of a strictly semi-honest strong Nash equilibrium; that is, one in which no voter puts any A above some B despite actually preferring B over A or being indifferent between the two.

If there is a majority Condorcet winner, there is almost certain to be a strong Nash equilibrium that favors that winner, in almost any reasonable deterministic voting system; but in some voting systems, that equilibrium may not be strictly semi-honest.

If there is a Condorcet winner but not a majority Condorcet winner (in other words, if enough voters are indifferent between the CW X and some other candidate Y, so that the social preference for X over Y is not a majority), it may not be possible to have a strictly semi-honest strong Nash equilibrium in a candidate-blind, non-dictatorial voting system.

Smith set equilibrium
Many voting methods that have an equilibrium around the Condorcet winner likely more generally have an equilibrium around any candidate in the Smith set, particularly if every candidate in the Smith set majority-beats all candidates not in the Smith set. For example, with Approval voting: 2: A>B>C

2: B>C>A

2: C>A>B

5: D A, B, and C are in the "majority Smith set" (every candidate in the Smith set majority-beat s every candidate not in the set). Every voter in the (A, B, C) solid coalition has an incentive to approve all of (A, B, C) to ensure that one of them wins, rather than D; if any of them approve fewer candidates, then D wins or at least ties, which is strictly worse from the solid coalition voters' perspectives.

Another example:

A through E are in the Smith set (there are beat-or-tie path s of A<C<B and C<D=E). Suppose F wins in Score voting; then the majority that prefers any of the Smith set members can set their approval threshold between the Smith set member and F, giving the Smith set member a majority of points and F a minority of points.

Strong Nash equilibrium
Applied to voting theory, a strong Nash equilibrium means a set of votes, where no coalition of voters can change their votes to get a result they all prefer. This is one of the strongest, most elusive kinds of equilibria in voting theory. The only ways to make it stronger are if it is known (through some reliable aspect of the system, not just through polling) and/or unique. It has also been called a coalition-proof social equilibrium or CPSE.