 Solution

Write $x(S)={\sum }_{i\in S}{x}_i$. The core of a cooperative game isTake any $x,y\in C(v)$ and $\alpha \in [0,1]$, and form the convex combination $z=\alpha x+(1-\alpha )y$. Its total payoff is $z(N)=\alpha v(N)+(1-\alpha )v(N)=v(N)$. For every coalition $S$,Hence $z$ remains in the core. The core is a convex set, even when the transferable utility game is not a convex cooperative game. An empty core is a convex set vacuously; the argument does not claim nonemptiness. Geometrically, the same conclusion follows because the constraints intersect an affine hyperplane with closed half-spaces.