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Proizvolov's Identity


Proizvolov's identity concerns a set partition of the integers 1, 2, ..., 2n into two n-element sets. Write one set in increasing order a_1<a_2<...<a_n and the other in decreasing order b_1>b_2>...>b_n. The identity states that

 sum_(i=1)^n|a_i-b_i|=n^2.

For each i, at least one of a_i and b_i exceeds n. Otherwise the first i terms of the increasing list and the last n-i+1 terms of the decreasing list would give n+1 distinct integers no greater than n. The pairwise maxima are therefore exactly n+1, n+2, ..., 2n, while the pairwise minima are 1, 2, ..., n. It follows that

 sum_(i=1)^n|a_i-b_i|=sum_(k=n+1)^(2n)k-sum_(k=1)^nk=n^2.

See also

Absolute Value, Permutation, Set Partition

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References

Bogomolny, A. "Proizvolov's Identity." https://cut-the-knot.org/Curriculum/Games/ProizvolovGame.shtml.Savchev, S. and Andreescu, T. "The 'Arbitrary' Proizvolov." Ch. 18 in Mathematical Miniatures. Washington, DC: Mathematical Association of America, pp. 66-69, 2003.

Cite this as:

Weisstein, Eric W. "Proizvolov's Identity." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/ProizvolovsIdentity.html

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