Finding nucleolus of flow game

We study the algorithmic issues of finding the nucleolus of a flow game. The flow game is a cooperative game defined on a network D=(V,E;ω). The player set is E and the value of a coalition S⊆E is defined as the value of a maximum flow from source to sink in the subnetwork induced by S. We show that the nucleolus of the flow game defined on a simple network (ω(e)=1 for each e∈E) can be computed in polynomial time by a linear program duality approach, settling a twenty-three years old conjecture by Kalai and Zemel. In contrast, we prove that both the computation and the recognition of the nucleolus are -hard for flow games with general capacity. Supported by NCET, NSFC (10771200), a CERG grant (CityU 1136/04E) of Hong Kong RGC, an SRG grant (7001838) of City University of Hong Kong.