Addresses:  
 
Department of Mathematics, Heriot-Watt University, Edinburgh EH14 4AS,  
Scotland, UK; Steklov Institute of Mathematics, 8 Gubkina St.,  
117966 Moscow, Russia; e-mail: S.B.Kuksin@ma.hw.ac.uk 
 
Laboratoire de Mathematiques, Universite de Paris--Sud XI,  
Batiment 425, 91405 Orsay Cedex, France;  
e-mail: Armen.Shirikyan@math.u-psud.fr