Institute of Mathematics and Informatics Bulgarian Academy of Sciences
Serdica Mathematical Journal, Vol. 34, No 1, (2008), 55p-66p
The problem studied here was suggested to us by V. Petkov.
Since the beginning of our careers, we have benefitted from his insights in
partial differential equations and mathematical physics. In his writings and
many discussions, the conjuction of deep analysis and specially interesting
problems has been a source inspiration for us.