Copyright 2011 by Instytut Matematyczny PAN. Article originally published in Acta arithmetica, Vol. 148, No.1, pp. 93-103. The original article can be found at http://dx.doi.org/10.4064/aa148-1-7. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.