New bounds on randomized metric distortion of top-$k$ voting
We prove new upper and lower bounds on metric distortion for randomized social choice mechanisms. Under first-choice voting where each voter reports only their most preferred candidate, we show that selecting a candidate with probability proportional to the $\frac{n}{n-1}$-th power of their vote share achieves the optimal worst-case distortion of $3 - \frac{2}{n}$. This is a simpler single-rule alternative to prior work. We also study instance-specific metric distortion of first-choice mechanisms in terms of the vote vector $\nu$. We show that there is a uniquely optimal rule achieving distortion $1 + \frac{2}{\sum_i \frac{\nu_i}{1 - \nu_i}}$. Finally, we extend our results to top-$k$ voting where each voter reports their $k$ nearest candidates. We derive a formula for the worst-case distortion for any $k\ge 2$. For the cyclic profile family this improves the previously best known $3 - \frac{2}{\lfloor \frac{n}{k} \rfloor}$ lower bound.