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A063865
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Number of solutions to +- 1 +- 2 +- 3 +- ... +- n = 0.
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48
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1, 0, 0, 2, 2, 0, 0, 8, 14, 0, 0, 70, 124, 0, 0, 722, 1314, 0, 0, 8220, 15272, 0, 0, 99820, 187692, 0, 0, 1265204, 2399784, 0, 0, 16547220, 31592878, 0, 0, 221653776, 425363952, 0, 0, 3025553180, 5830034720, 0, 0, 41931984034, 81072032060, 0, 0
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OFFSET
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0,4
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COMMENTS
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Number of sum partitions of the half of the n-th-triangular number by distinct numbers in the range 1 to n. Example: a(7)=8 since triangular(7)=28 and 14 = 2+3+4+5 = 1+3+4+6 = 1+2+5+6 = 3+5+6 = 7+1+2+4 = 7+3+4 = 7+2+5 = 7+1+6. - Hieronymus Fischer, Oct 20 2010
The asymptotic formula below was stated as a conjecture by Andrica & Tomescu in 2002 and proved by B. D. Sullivan in 2013. See his paper and H.-K. Hwang's review MR 2003j:05005 of the JIS paper. - Jonathan Sondow, Nov 11 2013
a(n) is the number of subsets of {1..n} whose sum is equal to the sum of their complement. See example below. - Gus Wiseman, Jul 04 2019
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LINKS
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T. D. Noe, N. J. A. Sloane and Ray Chandler, Table of n, a(n) for n = 0..3339 (terms < 10^1000, first 101 terms from T. D. Noe, next 300 terms from N. J. A. Sloane)
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FORMULA
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Asymptotic formula: a(n) ~ sqrt(6/Pi)*n^(-3/2)*2^n for n = 0 or 3 (mod 4) as n approaches infinity.
a(n) = 0 unless n == 0 or 3 (mod 4).
a(n) = constant term in expansion of Product_{ k = 1..n } (x^k + 1/x^k). - N. J. A. Sloane, Jul 07 2008
If n = 0 or 3 (mod 4) then a(n) = coefficient of x^(n(n+1)/4) in Product_{k=1..n} (1+x^k). - D. Andrica and I. Tomescu.
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EXAMPLE
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For example, the a(0) = 1 through a(8) = 14 subsets (empty columns not shown) are:
{} {3} {1,4} {1,6,7} {3,7,8}
{1,2} {2,3} {2,5,7} {4,6,8}
{3,4,7} {5,6,7}
{3,5,6} {1,2,7,8}
{1,2,4,7} {1,3,6,8}
{1,2,5,6} {1,4,5,8}
{1,3,4,6} {1,4,6,7}
{2,3,4,5} {2,3,5,8}
{2,3,6,7}
{2,4,5,7}
{3,4,5,6}
{1,2,3,4,8}
{1,2,3,5,7}
{1,2,4,5,6}
(End)
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MAPLE
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M:=400; t1:=1; lprint(0, 1); for n from 1 to M do t1:=expand(t1*(x^n+1/x^n)); lprint(n, coeff(t1, x, 0)); od: # N. J. A. Sloane, Jul 07 2008
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MATHEMATICA
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f[n_, s_] := f[n, s]=Which[n==0, If[s==0, 1, 0], Abs[s]>(n*(n+1))/2, 0, True, f[ n-1, s-n]+f[n-1, s+n]]; a[n_] := f[n, 0]
nmax = 50; d = {1}; a1 = {};
Do[
i = Ceiling[Length[d]/2];
AppendTo[a1, If[i > Length[d], 0, d[[i]]]];
d = PadLeft[d, Length[d] + 2 n] + PadRight[d, Length[d] + 2 n];
, {n, nmax}];
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PROG
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(PARI) a(n)=0^n+floor(prod(k=1, n, 2^(n*k)+2^(-n*k)))%(2^n) \\ Tani Akinari, Mar 09 2016
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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