for the initial and initial-boundary value problems. A
solution, u, is said to be self-similar if
for al l 
and 
. These are unique long-time
solutions. We require that the initial data, g, have small norm in
the homogeneous Besov space 
which can be
defined in terms of a Little wood-Payley decomposition. The existence
proof is obtained by iterating the appropriate equivalent integral
equation.