Ryser Formula
A formula for the Permanent of a Matrix
where the Sum is over all Subsets of
, and 
is the number ofelements in 
. The formula can be optimized by picking the Subsets so that only a single element ischanged at a time (which is precisely a Gray Code), reducing the number of additions from 
to 
.It turns out that the number of disks moved after the 
th step in the Towers of Hanoi is the same as the elementwhich needs to be added or deleted in the th Addend of the Ryser formula (Gardner 1988, Vardi 1991, p. 111).
References
Gardner, M. ``The Icosian Game and the Tower of Hanoi.'' Ch. 6 in The Scientific American Book of Mathematical Puzzles & Diversions. New York: Simon and Schuster, 1959. Knuth, D. E. The Art of Computer Programming, Vol. 2: Seminumerical Algorithms, 3rd ed. Reading, MA: Addison-Wesley, p. 515, 1998. Nijenhuis, A. and Wilf, H. Chs. 7-8 in Combinatorial Algorithms. New York: Academic Press, 1975. Vardi, I. Computational Recreations in Mathematica. Reading, MA: Addison-Wesley, p. 111, 1991.
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