We present a Levy-type characterization for time-homogeneous diffusion
processes on R of the following kind: A continuous process X = (X-t)(
t greater than or equal to 0) on R is a diffusion belonging to a secon
d-order differential operator L if and only if the two processes (p(X-
t))(t greater than or equal to 0) and (s(X-t)-t)(t greater than or equ
al to 0) are local martingales where the functions p, s are suitable s
olutions of Lp = 0 and Ls = 1.