See the slides:
 http://ii.fmph.uniba.sk/cl/view/courses/1-AIN-625-lpi/1314zs/pr/doc/handout.pr05.pdf
See also some of the lecture notes from previous years:
 http://ii.fmph.uniba.sk/cl/view/courses/1-AIN-625-lpp/0910zs/ln/doc/ch_p_cr.pdf