Let D be a subset of a Banach
space. Suppose R(t) : D → D (t ≧ 0) is an “almost-semigroup,” in the sense that
R(t)R(s) is close to R(t + s). If R also satisfies certain stability conditions, then
R(t∕n)n converges to some semigroup S(t) as n →∞. The stability conditions
are motivated by several examples involving nonlinear partial differential
equations.