The work of S P was supported in part by RFBR-14-01-00474-a. N A S was supported by the Program of RAS Nonlinear Dynamics in Mathematics and Physics, RFBR-14-01-00860-a, RFBR-13-01-12405-ofi-m2.
Поступила в редакцию: 21.06.2015 Принята в печать: 17.08.2015