求大神帮忙

A coalitional game with transferable payoff<N,v>  is simple if V(s) is either 0 or 1 for every coalition S ，and v(N)=1 ;a coalition S for which v(S)=1 is called a  winning coalitions. A player who belongs to all winning coalitions is a veto player.

a. Show that if there is no veto player then the core is empty;

b. Show that if the set of veto players is nonempty then the core is the set of all nonnegative feasible payoff profiles that give zero to all other players.