branching process

Gillespie DT 1977 Exact stochastic simulation of coupled chemical reactions. J Phys Chem 81:2340-2361.

• in returning to step1 from step 3, notice that it is necessary to recalculate only those quantities a_ν corresponding to reactions R_ν whose reactant population levels were just altered in step 3
• also a₀ may be recalculated simply by adding to a_ν the difference between each newly changed a_ν value and its corresponding old value