On Parameterized Complexity of Liquid Democracy

Palash Dey, Arnab Maiti, Amatya Sharma

Abstract: In liquid democracy, each voter either votes herself or delegates her vote to some other voter. This gives rise to what is called a delegation graph. To decide the voters who eventually votes along with the subset of voters whose votes they give, we need to resolve the cycles in the delegation graph. This gives rise to the Resolve Delegation to MinMaxWeight problem where we need to find an acyclic sub-graph of the delegation graph such that the number of voters whose votes they give is bounded above by some integer $$\lambda $$ . Putting a cap on the number of voters whose votes a voter gives enable the system designer restrict the power of any individual voter. The Resolve Delegation to MinMaxWeight problem is already known to be $$\mathsf {NP}$$ -hard. In this paper we study the parameterized complexity of this problem. We show that Resolve Delegation to MinMaxWeight is para- $$\mathsf {NP}\text {-hard}$$ with respect to parameters $$\lambda $$ , number of sink nodes and the maximum degree of the delegation graph. We also show that Resolve Delegation to MinMaxWeight is $$\mathsf {W[1]}$$ -hard even with respect to the treewidth of the delegation graph. We complement our negative results by exhibiting FPT algorithms with respect to some other parameters. We finally show that a related problem, which we call Resolve Fractional Delegation, is polynomial time solvable.