Two-Commodity Flow is Equivalent to Linear Programming under Nearly-Linear Time Reductions

TL;DR

This paper presents a nearly-linear time reduction from linear programming to the 2-commodity flow problem, establishing their near-equivalence and enabling efficient solutions for both using similar algorithms.

Contribution

It improves upon Itai's polynomial-time reduction by nearly preserving problem size and bounding errors, showing that 2-commodity flow and linear programming are nearly equivalent in computational complexity.

Findings

Reduction runs in nearly-linear time with small size blow-up

Approximate solutions to 2-commodity flow yield solutions to linear programs efficiently

Establishes near-equivalence between 2-commodity flow and linear programming

Abstract

We give a nearly-linear time reduction that encodes any linear program as a 2-commodity flow problem with only a small blow-up in size. Under mild assumptions similar to those employed by modern fast solvers for linear programs, our reduction causes only a polylogarithmic multiplicative increase in the size of the program and runs in nearly-linear time. Our reduction applies to high-accuracy approximation algorithms and exact algorithms. Given an approximate solution to the 2-commodity flow problem, we can extract a solution to the linear program in linear time with only a polynomial factor increase in the error. This implies that any algorithm that solves the 2-commodity flow problem can solve linear programs in essentially the same time. Given a directed graph with edge capacities and two source-sink pairs, the goal of the 2-commodity flow problem is to maximize the sum of the flows routed between the two source-sink pairs subject to edge capacities and flow conservation. A 2-commodity flow can be directly written as a linear program, and thus we establish a nearly-tight equivalence between these two classes of problems. Our proof follows the outline of Itai's polynomial-time reduction of a linear program to a 2-commodity flow problem (JACM'78). Itai's reduction shows that exactly solving 2-commodity flow and exactly solving linear programming are polynomial-time equivalent. We improve Itai's reduction to nearly preserve the problem representation size in each step. In addition, we establish an error bound for approximately solving each intermediate problem in the reduction, and show that the accumulated error is polynomially bounded. We remark that our reduction does not run in strongly polynomial time and that it is open whether 2-commodity flow and linear programming are equivalent in strongly polynomial time.