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A267240
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Number of n X 3 binary arrays with row sums nondecreasing and columns lexicographically nondecreasing.
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1
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4, 13, 42, 141, 486, 1685, 5804, 19769, 66544, 221581, 730918, 2391717, 7772610, 25110933, 80713016, 258280817, 823269116, 2615088973, 8281113730, 26150883901, 82375282494, 258893742933, 811984918692, 2541865829801, 7943330715176
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OFFSET
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1,1
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LINKS
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FORMULA
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Empirical: a(n) = 10*a(n-1) - 39*a(n-2) + 76*a(n-3) - 79*a(n-4) + 42*a(n-5) - 9*a(n-6).
G.f.: x*(4 - 27*x + 68*x^2 - 76*x^3 + 42*x^4 - 9*x^5) / ((1 - x)^4*(1 - 3*x)^2).
a(n) = (24 + (31+3^(2+n))*n + 12*n^2 + 2*n^3) / 24.
(End)
Empirical recurrence verified (see link). - Robert Israel, Sep 08 2019
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EXAMPLE
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Some solutions for n=4:
..0..0..1....0..0..1....0..0..0....0..0..1....0..0..1....0..0..1....0..0..1
..0..1..0....0..1..0....0..1..1....0..0..1....0..1..0....0..0..1....1..1..0
..1..0..0....1..1..0....1..0..1....0..1..1....0..0..1....0..0..1....0..1..1
..1..1..0....1..1..1....0..1..1....0..1..1....0..0..1....1..1..0....1..0..1
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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