Large systems of random linear equations with nonnegative solutions: Characterizing the solvable and the unsolvable...

Large systems of linear equations are ubiquitous in science. Quite often, e.g., when considering population dynamics or chemical networks, the solutions must be nonnegative. Recently, it has been shown that large systems of random linear equations exhibit a sharp transition from a phase, where a nonnegative solution exists with probability one, to one where typically no such solution may be found. The critical line separating the two phases was determined by combining Farkas' lemma with the replica method. Here we show that the same methods remain viable to characterize the two phases away from criticality. To this end we analytically determine the residual norm of the system in the unsolvable phase and a suitable measure of robustness of solutions in the solvable one. Our results are in very good agreement with numerical simulations.