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 A039924 G.f.: Sum_{k>=0} x^(k^2)*(-1)^k/(Product_{i=1..k} 1-x^i). 9
 1, -1, -1, -1, 0, 0, 1, 1, 2, 1, 2, 1, 1, 0, 0, -2, -1, -3, -3, -4, -3, -5, -3, -4, -2, -3, 0, -1, 3, 2, 5, 5, 9, 7, 11, 9, 13, 10, 13, 9, 12, 7, 9, 3, 5, -3, -1, -9, -7, -17, -15, -24, -21, -31, -27, -37, -31, -40, -33, -41, -31, -39, -27, -33, -18, -24, -6, -11, 9, 5, 26, 23 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,9 COMMENTS Ramanujan used the form Sum_{k>=0} x^(k^2) / (Product_{i=1..k} 1-(-x)^i), which is obtained by changing the sign of x. - Michael Somos, Jul 20 2003 Coefficients in expansion of determinant of infinite tridiagonal matrix shown below in powers of x^2 (Lehmer 1973):    1   x   0   0   0   0  ...    x   1  x^2  0   0   0  ...    0  x^2  1  x^3  0   0  ...    0   0  x^3  1  x^4  0  ...   ... ... ... ... ... ... ... Convolution inverse of A003116. REFERENCES N. J. Fine, Basic Hypergeometric Series and Applications, Amer. Math. Soc., 1988; p. 55, Eq. (26.11). G. H. Hardy, P. V. Seshu Aiyar and B. M. Wilson, editors, Collected Papers of Srinivasa Ramanujan, Cambridge, 1923; p. 354. D. H. Lehmer, Combinatorial and cyclotomic properties of certain tridiagonal matrices. Proceedings of the Fifth Southeastern Conference on Combinatorics, Graph Theory, and Computing (Florida Atlantic Univ., Boca Raton, Fla., 1974), pp. 53-74. Congressus Numerantium, No. X, Utilitas Math., Winnipeg, Man., 1974. MR0441852. Herman P. Robinson, personal communication to N. J. A. Sloane. LINKS Alois P. Heinz, Table of n, a(n) for n = 0..5000 (first 1001 terms from T. D. Noe) Shalosh B. Ekhad, Doron Zeilberger, D.H. Lehmer's Tridiagonal determinant: An Etude in (Andrews-Inspired) Experimental Mathematics, arXiv:1808.06730 [math.CO], 2018. Herman P. Robinson, Letter to N. J. A. Sloane, Nov 13 1973. FORMULA a(n) = A286041(n) - A286316(n) (conjectured). - George Beck, May 05 2017 Proof from Doron Zeilberger, Aug 20 2018: (Start) The generating  function for partitions whose parts differ by at least 2 with exactly k parts is (famously) q^(k^2)/((1-q)*...*(1-q^k)). Indeed, if you take any such partition and remove 1 from the smallest part, 3 from the second-smallest part, etc., you remove 1+3+...+(2k-1) = k^2 and are left with an ordinary partition whose number of parts is <= k whose generating function is 1/((1-q)*...*(1-q)^k). Summing these up famously gives the generating function for partitions whose differences is >= 2. Sticking a (-1)^k in front gives the generating function for the difference between such partitions with an even number of parts and an odd number of parts, since (-1)^even=1 and (-1)^odd=-1. (End) EXAMPLE G.f. = 1 - x - x^2 - x^3 + x^6 + x^7 + 2*x^8 + x^9 + 2*x^10 + x^11 + x^12 + ... MAPLE qq:=n->mul( 1-(-q)^i, i=1..n); add (q^(n^2)/qq(n), n=0..100): series(t1, q, 99); MATHEMATICA CoefficientList[ Series[ Sum[x^k^2*(-1)^k / Product[1-x^i, {i, 1, k}], {k, 0, 100}], {x, 0, 100}], x][[1 ;; 72]] (* Jean-François Alcover, Apr 08 2011 *) a[ n_] := If[n < 0, 0, SeriesCoefficient[ Sum[ (-1)^k x^k^2 / QPochhammer[ x, x, k], {k, 0, Sqrt[n]}], {x, 0, n}]] (* Michael Somos, Jan 04 2014 *) PROG (PARI) {a(n) = if( n<0, 0, polcoeff( sum(k=0, sqrtint(n), x^k^2 / prod(i=1, k, x^i - 1, 1 + x * O(x^n))), n))} /* Michael Somos, Jul 20 2003 */ CROSSREFS Cf. A003116, A224898. Sequence in context: A093654 A342627 A220115 * A349218 A275346 A037888 Adjacent sequences:  A039921 A039922 A039923 * A039925 A039926 A039927 KEYWORD sign,nice AUTHOR EXTENSIONS More terms from Vladeta Jovovic, Mar 05 2001 STATUS approved

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Last modified August 18 19:27 EDT 2022. Contains 356215 sequences. (Running on oeis4.)