Conditions are given on a
nonlinear operator A in a Banach space X under which the semigroup, S(t),
generated by −A has the property that S(t)x is analytic in t for |argt| < 𝜃 for each
fixed x ∈cl(D(A)). Analyticity in t of solutions of u′ + Tu = Fu where −T generates
a linear holomorphic semigroup in X and F maps D(Tα) analytically into X
for some α < 1 is also established. These results are applied to establish
analyticity in t of solutions to ∂u∕∂t + Lu + β(u) = 0 where β : R → R is real
analytic, monotone increasing and β(0) = 0, and L is a second order elliptic
operator.