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A063865 Number of solutions to +- 1 +- 2 +- 3 +- ... +- n = 0. 42
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

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

LINKS

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)

D. Andrica and E. J. Ionascu, Variations on a result of Erdős and Surányi, INTEGERS 2013 slides.

D. Andrica and I. Tomescu, On an Integer Sequence Related to a Product of Trigonometric Functions, and Its Combinatorial Relevance, J. Integer Seq., 5 (2002), Article 02.2.4

Ovidiu Bagdasar and Dorin Andrica, New results and conjectures on 2-partitions of multisets, 2017 7th International Conference on Modeling, Simulation, and Applied Optimization (ICMSAO).

Steven R. Finch, Signum equations and extremal coefficients, February 7, 2009. [Cached copy, with permission of the author]

B. D. Sullivan, On a Conjecture of Andrica and Tomescu, J. Int. Sequences, 16 (2013), Article 13.3.1.

zbMATH, Review of Andrica and Tomescu

FORMULA

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.

a(n) = 2*A058377(n) for any n > 0. - Rémy Sigrist, Oct 11 2017

EXAMPLE

From Gus Wiseman, Jul 04 2019: (Start)

a(n) is the number of subsets of {1..n} whose sum is equal to the sum of their complement. 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)

MAPLE

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

MATHEMATICA

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}];

a1 (* Ray Chandler, Mar 13 2014 *)

PROG

(PARI) a(n)=my(x='x); polcoeff(prod(k=1, n, x^k+x^-k)+O(x), 0) \\ Charles R Greathouse IV, May 18 2015

(PARI) a(n)=0^n+floor(prod(k=1, n, 2^(n*k)+2^(-n*k)))%(2^n) \\ Tani Akinari, Mar 09 2016

CROSSREFS

Cf. A000980, A025591, A058377, A063866, A063867, A113036, A113037, A141000.

"Decimations": A060468 = 2*A060005, A123117 = 2*A104456.

Analogous sequences for sums of squares and cubes are A158092, A158118, see also A019568. - Pietro Majer, Mar 15 2009

Cf. A053632, A059529, A326173, A326174, A326175.

Sequence in context: A320525 A069971 A167291 * A230275 A230592 A282699

Adjacent sequences:  A063862 A063863 A063864 * A063866 A063867 A063868

KEYWORD

nonn,easy,nice,changed

AUTHOR

N. J. A. Sloane, suggested by J. H. Conway, Aug 27 2001

EXTENSIONS

More terms from Dean Hickerson, Aug 28 2001

Corrected and edited by Steven Finch, Feb 01 2009

STATUS

approved

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Last modified July 17 18:47 EDT 2019. Contains 325109 sequences. (Running on oeis4.)