On O ( n ) algorithms for projection onto the top- k -sum sublevel set.

The top-k-sum operator computes the sum of the largest $k$ components of a given vector. The Euclidean projection onto the top- $k$ -sum sublevel set serves as a crucial subroutine in iterative methods to solve composite superquantile optimization problems. In this paper, we introduce a solver that implements two finite-termination algorithms to compute this projection. Both algorithms have $O\left(n\right)$ complexity of floating point operations when applied to a sorted $n$ -dimensional input vector, where the absorbed constant is independent of $k$ . This stands in contrast to an existing grid-search-inspired method that has $O\left(k\right(n-k\left)\right)$ complexity, a partition-based method with $O(n+D\text{log}D)$ complexity, where $D\le n$ is the number of distinct elements in the input vector, and a semismooth Newton method with a finite termination property but unspecified floating point complexity. The improvement of our methods over the first method is significant when $k$ is linearly dependent on $n$ , which is frequently encountered in practical superquantile optimization applications. In instances where the input vector is unsorted, an additional cost is incurred to (partially) sort the vector, whereas a full sort of the input vector seems unavoidable for the other two methods. To reduce this cost, we further derive a rigorous procedure that leverages approximate sorting to compute the projection, which is particularly useful when solving a sequence of similar projection problems. Numerical results show that our methods solve problems of scale $n={10}^7$ and $k={10}^4$ within 0.05 s, whereas the most competitive alternative, the semismooth Newton-based method, takes about 1 s. The existing grid-search method and Gurobi's QP solver can take from minutes to hours.