Extended soliton solutions for the Kaup-Kupershmidt equation

We extend the N-soliton solutions of the Kaup-Kupershmidt equation on a nonzero background decreasing as (x + 1/a)(-2). These new solutions describe the interaction of N solitary waves with a static bell-shaped wave. We give the conditions so that the Backlund transformation relating those solutions and the N-solitons of the Sawada-Kotera equation will be satisfied.