Subgame Perfect Equilibrium

Subgame Perfect Equilibrium is information set for a player is a collection of decision nodes satisfying that- (i) the player has the move at every node in the information set; and (ii) when the play of the game reaches a node in the information set, the player with the move does not know which node in the information set has been reached. A subgame in an extensive-form game (a) begins at some decision node n with a singleton information set, (b) includes all the decision and terminal nodes following n, and (c) does not cut any information sets.