http://www.cut-the-knot.org/Probability/PGF.shtml [2]
http://www.cut-the-knot.org/arithmetic/combinatorics/Sicherman.shtml [3]
Links[1] https://www.hootsgo.org/?q=node/6722 [2] http://www.cut-the-knot.org/Probability/PGF.shtml [3] http://www.cut-the-knot.org/arithmetic/combinatorics/Sicherman.shtml
