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A324631
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Number of permutations p of [n] such that four is the maximum of the number of elements in any integer interval [p(i)..i+n*[i<p(i)]].
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3
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15, 31, 60, 113, 215, 406, 763, 1431, 2676, 4993, 9299, 17290, 32103, 59535, 110292, 204137, 377535, 697742, 1288763, 2379167, 4390148, 8097681, 14931075, 27522586, 50719103, 93444207, 172125100, 316999057, 583718215, 1074702870, 1978430491, 3641722423
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OFFSET
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4,1
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LINKS
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FORMULA
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G.f.: -x^4*(10*x^4+23*x^3+17*x^2-x-15)/((x^2+x-1)*(x^3+x^2+x-1)).
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EXAMPLE
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a(4) = 15: 1243, 1324, 1342, 2134, 2143, 2314, 2341, 2413, 2431, 3142, 3241, 3421, 4231, 4312, 4321.
a(5) = 31: 12534, 12543, 14235, 14325, 14523, 14532, 15342, 31245, 31524, 31542, 32145, 32514, 34125, 34215, 34512, 35124, 35142, 35214, 41523, 41532, 42315, 42513, 45132, 45213, 45312, 51342, 52314, 54123, 54132, 54213, 54312.
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MAPLE
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a:= n-> `if`(n<4, 0, (<<0|1|0|0|0>, <0|0|1|0|0>, <0|0|0|1|0>,
<0|0|0|0|1>, <-1|-2|-1|1|2>>^n. <<4, 1, 3, 10, 15>>)[1$2]):
seq(a(n), n=4..40);
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MATHEMATICA
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LinearRecurrence[{2, 1, -1, -2, -1}, {15, 31, 60, 113, 215}, 40] (* Vincenzo Librandi, Jun 06 2019 *)
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PROG
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(Magma) I:=[15, 31, 60, 113, 215]; [n le 5 select I[n] else 2*Self(n-1)+Self(n-2)-Self(n-3)-2*Self(n-4)+Self(n-5): n in [1..40]]; // Vincenzo Librandi, Jun 06 2019
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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