XX model on the circle A. De Pasquale, G. Costantini, P. Facchi, G. Florio, S. Pascazio and K. Yuasa Eur. Phys. J. Special Topics, 160 (2008) 127-138 Published online: 18 June 2008 DOI: 10.1140/epjst/e2008-00716-9