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Monge array : ウィキペディア英語版
Monge array

In mathematics applied to computer science, Monge arrays, or Monge matrices, are mathematical objects named for their discoverer, the French mathematician Gaspard Monge.
An ''m''-by-''n'' matrix is said to be a ''Monge array'' if, for all \scriptstyle i,\, j,\, k,\, \ell such that
:1\le i < k\le m\text1\le j < \ell\le n
one obtains
:A() + A() \le A() + A().\,
So for any two rows and two columns of a Monge array (a 2 × 2 sub-matrix) the four elements at the intersection points have the property that the sum of the upper-left and lower right elements (across the main diagonal) is less than or equal to the sum of the lower-left and upper-right elements (across the antidiagonal).
This matrix is a Monge array:
:
\begin
10 & 17 & 13 & 28 & 23 \\
17 & 22 & 16 & 29 & 23 \\
24 & 28 & 22 & 34 & 24 \\
11 & 13 & 6 & 17 & 7 \\
45 & 44 & 32 & 37 & 23 \\
36 & 33 & 19 & 21 & 6 \\
75 & 66 & 51 & 53 & 34 \end
For example, take the intersection of rows 2 and 4 with columns 1 and 5.
The four elements are:
:
\begin
17 & 23\\
11 & 7 \end
: 17 + 7 = 24
: 23 + 11 = 34
The sum of the upper-left and lower right elements is less than or equal to the sum of the lower-left and upper-right elements.
==Properties==

*The above definition is equivalent to the statement
:A matrix is a Monge array if and only if A() + A()\le A() + A() for all 1\le i < m and 1\le j < n.
*Any subarray produced by selecting certain rows and columns from an original Monge array will itself be a Monge array.
*Any linear combination with non-negative coefficients of Monge arrays is itself a Monge array.
*One interesting property of Monge arrays is that if you mark with a circle the leftmost minimum of each row, you will discover that your circles march downward to the right; that is to say, if f(x) = \arg\min_{i\in \{1,\ldots,m\}} A(), then f(j)\le f(j+1) for all 1\le j < n. Symmetrically, if you mark the uppermost minimum of each column, your circles will march rightwards and downwards. The row and column ''maxima'' march in the opposite direction: upwards to the right and downwards to the left.
*The notion of ''weak Monge arrays'' has been proposed; a weak Monge array is a square ''n''-by-''n'' matrix which satisfies the Monge property A() + A()\le A() + A() only for all 1\le i < r,s\le n.
*Every Monge array is totally monotone, meaning that its row minima occur in a nondecreasing sequence of columns, and that the same property is true for every subarray. This property allows the row minima to be found quickly by using the SMAWK algorithm.
*Monge matrix is just another name for submodular function of two discrete variables. Precisely, ''A'' is a Monge matrix if and only if ''A''() is a submodular function of variables ''i'',''j''.

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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