Reachability problems for continuous chemical reaction networks

Abstract

Chemical reaction networks (CRNs) model the behavior of molecules in a well-mixed solution. The emerging field of molecular programming uses CRNs not only as a descriptive tool, but as a programming language for chemical computation. Recently, Chen, Doty and Soloveichik introduced rate-independent continuous CRNs (CCRNs) to study the chemical computation of continuous functions. A fundamental question for any CRN model is reachability, the question whether a given target state is reachable from a given start state via a sequence of reactions (a path) in the network. In this paper, we investigate CCRN-REACH, the reachability problem for rate-independent continuous chemical reaction networks. Our main theorem is that, for CCRNs, deciding reachability—and constructing a path if there is one—is computable in polynomial time. This contrasts sharply with the known exponential space hardness of the reachability problem for discrete CRNs. We also prove that the related problem Sub-CCRN-REACH, which asks about reachability in a CCRN using only a given number of its reactions, is NP-complete.