C. Michael, G. Tickle, and M. Teper. Physics Letters B, 207 (3):
313 - 318(1988)http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&sqi=2&ved=0CFAQFjAB&url=http%3A%2F%2Fusers.iit.demokritos.gr%2F~sper%2FPAPERS%2F1992J-MichaelPerantonis-JPG.pdf&ei=5hTHT_KECMjHtAaV47XhDg&usg=AFQjCNGJDqylpFEsXVlgJJ09-8G4tnsGGQ.