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Proizvolov's identity : ウィキペディア英語版
Proizvolov's identity
In mathematics, Proizvolov's identity is an identity concerning sums of differences of positive integers. The identity was posed by Vyacheslav Proizvolov as a problem in the 1985 All-Union Soviet Student Olympiads .
To state the identity, take the first 2''N'' positive integers,
:1, 2, 3, ..., 2''N'' − 1, 2''N'',
and partition them into two subsets of ''N'' numbers each. Arrange one subset in increasing order:
: A_1 < A_2 < \cdots < A_N.
Arrange the other subset in decreasing order:
: B_1 > B_2 > \cdots > B_N.
Then the sum
: |A_1-B_1| + |A_2-B_2| + \cdots + |A_N-B_N|
is always equal to ''N''2.
==Example==
Take for example ''N'' = 3. The set of numbers is then . Select three numbers of this set, say 2, 3 and 5. Then the sequences ''A'' and ''B'' are:
:''A''1 = 2, ''A''2 = 3, and ''A''''3'' = 5;
:''B''1 = 6, ''B''2 = 4, and ''B''''3'' = 1.
The sum is
:|A_1-B_1| + |A_2-B_2| + |A_3-B_3| = |2-6| + |3-4| + |5-1| = 4+1+4 = 9,
which indeed equals 32.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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