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A070214
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Maximal number of occupied cells in all monotonic matrices of order n.
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4
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1, 2, 5, 8, 11, 14, 19, 23, 28, 32, 38, 43, 49, 55
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OFFSET
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1,2
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COMMENTS
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A monotonic matrix of order n is an n X n matrix in which every cell contains 0 or 1 numbers from the set {1...n} subject to 3 conditions:
(1) The filled-in entries in each row are strictly increasing;
(2) The filled-in entries in each column are strictly decreasing;
(3) For two filled-in cells with same entry, the one further right is higher (the positive slope condition).
From Rob Pratt: The problem can be formulated as a maximum independent set problem in a graph with n^3 nodes (i, j, k) in {1, 2, ..., n}^3. If node (i, j, k) appears in the solution, the interpretation is that cell (i, j) should contain k. The arcs, which indicate conflicting choices, are as follows:
Arc joining (I1, j1, k1) and (i2, j2, k2) if:
[rows increasing] i1 = i2 and ((j1 < j2 and k1 >= k2) or (j1 > j2 and k1 <= k2)).
[columns decreasing] j1 = j2 and ((i1 < i2 and k1 <= k2) or (i1 > i2 and k1 >= k2)).
[one color per cell] i1 = i2 and j1 = j2 and k1 <> k2.
[positive slope] k1 = k2 and i1 <> i2 and (j2 - j1) / (i2 - i1) > 0.
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LINKS
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FORMULA
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a(r*s) >= a(r)*a(s); if a(n) = n^e(n) then L := lim_{n->infinity} e(n) exists and is in the range 1.513 <= L <= 2.
Tiskin showed that a(n) = o(n^2).
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EXAMPLE
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a(3) >= 5 from this matrix:
2 - 3
- - 1
1 3 -
a(5) >= 11 from this matrix:
- - 4 - 5
4 - - 5 -
- - 1 2 3
3 5 - - -
1 2 - - -
Dean Hickerson found the following matrix, which improves the lower bound for a(8) to 23: (This is now known to be optimal)
- - 2 - - 4 7 8
- - 1 7 8 - - -
7 8 - - - - - -
- 2 - 4 - - - 6
- 1 - - - 3 6 -
4 - - - 6 - - -
2 - - - 3 - - 5
1 - - 3 - - 5 -
Paul Jungeblut improves the lower bound for a(11) to 38 with this matrix.
-- -- 8 -- -- -- 9 -- -- -- 11
-- 8 -- -- -- 9 -- -- -- -- 10
8 -- -- -- 9 -- -- -- 10 11 --
-- -- -- -- -- -- -- -- 4 5 7
-- 4 -- -- -- 5 7 11 -- -- --
-- -- -- -- -- 1 -- -- 2 3 6
4 -- -- -- 5 -- 6 10 -- -- --
-- -- -- -- 1 -- 2 3 -- -- --
2 3 7 11 -- -- -- -- -- -- --
-- 1 6 10 -- -- -- -- -- -- --
1 -- 5 9 -- -- -- -- -- -- --
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CROSSREFS
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KEYWORD
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nonn,more,hard,nice
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AUTHOR
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EXTENSIONS
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a(1)-a(5) computed by K. Joy. a(6) = 14 was established by Szabo.
Jul 27 2003 - Aug 23 2003: Rob Pratt has used integer programming to confirm the values for n <= 6 and has shown that a(7) = 19, 23 <= a(8) <= 28, 28 <= a(9) <= 42 and 32 <= a(10) <= 62.
Extended to a(14) from Tiskin (2007), who gives a(15) >= 61, a(16) >= 65.
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STATUS
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approved
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