Before revision:  We recall that a fully open network (alternatively a fully open CFSTR) is a chemical reaction network where all chemical species participate in the inflow and the outflow; and a CFSTR atom of multistationarity N is a multistationary fully open network such that the operation of `removing reactions' or `removing species' destroys the multistationarity of N. The class of CFSTR atoms of multistationarity determines the entire class of multistationary fully open networks since possessing a CFSTR atom of multistationarity as an `embedded network' (N is embedded in G if N can be obtained from G by removing a finite subset of reactions and species) was proven recently to be a sufficient condition for a fully open network to admit multistationarity.
After revision:  We recall that a fully open network (alternatively a fully open CFSTR) is a chemical reaction network where all chemical species participate in inflow and outflow, containing one non-flow (reversible or irreversible) reaction. We show that such a network admits multiple positive mass-action steady states if and only if the stoichiometric coefficients in the non-flow reaction satisfy a certain simple arithmetic relation. The multistationary fully open networks since possessing a CFSTR atom of multistationarity as an `embedded network' (N is embedded in G if N can be obtained from G by removing a finite subset of reactions and species) was proven recently to be a sufficient condition for a fully open network to admit multistationarity.