  We establish Dahlberg's perturbation theorem for non-divergence form
operators L = A\nabla^2. If L_0 and L_1 are two operators on a Lipschitz domain
such that the L^p Dirichlet problem for the operator L_0 is solvable for some p
in (1,\infty) and the coefficients of the two operators are sufficiently close
in the sense of Carleson measure, then the L^p Dirichlet problem for the
operator L_1 is solvable for the same p. This is an improvement of the
A_{\infty} version of this result proved by Rios in "The L^p Diriclet problem
and nondivergence harmonic measure" (Trans. AMS 355, 2 (2003)). As a
consequence we also improve a result from Dindos, Petermichl and Pipher, "The
L^p Dirichlet problem for second order elliptic operators and a p-adapted
square function" (J. Fun. Anal. 249 (2007)) for the L^p solvability of
non-divergence form operators by substantially weakening the condition required
on the coefficients of the operator. The improved condition is exactly the same
one as is required for divergence form operators L = div A\nabla.
