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A202261
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Number of n-element subsets that can be chosen from {1,2,...,2*n} having element sum n^2.
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4
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1, 1, 1, 3, 7, 18, 51, 155, 486, 1555, 5095, 17038, 57801, 198471, 689039, 2415043, 8534022, 30375188, 108815273, 392076629, 1420064031, 5167575997, 18885299641, 69287981666, 255121926519, 942474271999, 3492314839349, 12977225566680, 48349025154154
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OFFSET
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0,4
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COMMENTS
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a(n) is the number of partitions of n^2 into n distinct parts <= 2*n.
Taking the complement of each set, a(n) is also the number of partitions of n^2+n into n distinct parts <= 2*n. - Franklin T. Adams-Watters, Jan 20 2012
Also the number of partitions of n*(n+1)/2 into at most n parts of size at most n. a(4) = 7: 433, 442, 3322, 3331, 4222, 4321, 4411. - Alois P. Heinz, May 31 2020
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LINKS
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FORMULA
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EXAMPLE
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a(0) = 1: {}.
a(1) = 1: {1}.
a(2) = 1: {1,3}.
a(3) = 3: {1,2,6}, {1,3,5}, {2,3,4}.
a(4) = 7: {1,2,5,8}, {1,2,6,7}, {1,3,4,8}, {1,3,5,7}, {1,4,5,6}, {2,3,4,7},{2,3,5,6}.
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MAPLE
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b:= proc(n, i, t) option remember;
`if`(i<t or n<t*(t+1)/2 or n>t*(2*i-t+1)/2, 0,
`if`(n=0, 1, b(n, i-1, t) +`if`(n<i, 0, b(n-i, i-1, t-1))))
end:
a:= n-> b(n^2, 2*n, n):
seq(a(n), n=0..30);
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MATHEMATICA
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b[n_, i_, t_] := b[n, i, t] = If[i<t || n<t*(t+1)/2 || n>t*(2*i-t+1)/2, 0, If[n == 0, 1, b[n, i-1, t] + If[n<i, 0, b[n-i, i-1, t-1]]]]; a[n_] = b[n^2, 2*n, n]; Table[a[n], {n, 0, 30}] (* Jean-François Alcover, Feb 05 2015, after Alois P. Heinz *)
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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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