The PPP-completeness of the Ward-Szabo theorem
For complexity theorists, this establishes the exact computational complexity of a previously open problem, showing it is PPP-complete.
The paper proves that the Ward-Szabó problem, a total search problem of finding a bichromatic triangle in a colored complete graph, is complete for the complexity class PPP, improving upon previous PWPP-hardness results.
Ward and Szabó [WS94] have shown that a complete graph with $N^2$ nodes whose edges are colored by $N$ colors and that has at least two colors contains a bichromatic triangle. This fact leads us to a total search problem: Given an edge-coloring on a complete graph with $N^2$ nodes using at least two colors and at most $N$ colors, find a bichromatic triangle. Bourneuf, Folwarczný, Hubácek, Rosen, and Schwartzbach [Bou+23] have proven that such a total search problem, called Ward-Szabó, is PWPP-hard and belongs to the class TFNP, a class for total search problems in which the correctness of every candidate solution is efficiently verifiable. However, it is open which TFNP subclass contains Ward-Szabó. This paper will improve the computational complexity of Ward-Szabó. We prove that Ward-Szabó is a complete problem for the complexity class PPP, a TFNP subclass of problems in which the existence of solutions is guaranteed by the pigeonhole principle.