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 A089801 a(n) = 0 unless n = 3j^2+2j or 3j^2+4j+1 for some j>=0, in which case a(n) = 1. 22
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 OFFSET 0,1 COMMENTS Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700). Also characteristic function of generalized octagonal numbers A001082. - Omar E. Pol, Jul 13 2012 Number 12 of the 14 primitive eta-products which are holomorphic modular forms of weight 1/2 listed by D. Zagier on page 30 of "The 1-2-3 of Modular Forms". - Michael Somos, May 04 2016 LINKS Seiichi Manyama, Table of n, a(n) for n = 0..1000 George E. Andrews, The Bhargava-Adiga Summation and Partitions, 2016. See Th. 2. S. Cooper and M. D. Hirschhorn, Results of Hurwitz type for three squares. Discrete Math. 274 (2004), no. 1-3, 9-24. See X(q). Eric Weisstein's World of Mathematics, Ramanujan Theta Functions Eric Weisstein's World of Mathematics, Jacobi Theta Functions I. J. Zucker, Further Relations Amongst Infinite Series and Products. II. The Evaluation of Three-Dimensional Lattice Sums, J. Phys. A: Math. Gen. 23, 117-132, 1990. FORMULA G.f.: Sum_{n=-infty ..infty } q^(3n^2+2n). Expansion of Jacobi theta function (theta_3(q^(1/3)) - theta_3(q^3))/(2 q^(1/3)) in powers of q. Euler transform of period 12 sequence [1, -1, 0, 0, 1, -1, 1, 0, 0, -1, 1, -1, ...]. - Michael Somos, Apr 12 2005 a(n) = b(3*n + 1) where b() is multiplicative with b(3^e) = 0^e, b(p^e) = (1 + (-1)^e) / 2 if p != 3. - Michael Somos, Jun 06 2005; b=A033684. - R. J. Mathar, Oct 07 2011 Expansion of q^(-1/3) * eta(q^2)^2 * eta(q^3) * eta(q^12) / (eta(q) * eta(q^4) * eta(q^6)) in powers of q. - Michael Somos, Apr 12 2005 Expansion of chi(x) * psi(-x^3) in powers of x where chi(), psi() are Ramanujan theta functions. - Michael Somos, Apr 19 2007 Expansion of f(x, x^5) in powers of x where f(, ) is Ramanujan's general theta function. - Michael Somos, Jun 29 2012 G.f. is a period 1 Fourier series which satisfies f(-1 / (36 t)) = 2^(1/2) (t/i)^(1/2) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A089807. a(8*n + 4) = a(4*n + 2) = a(4*n + 3) = 0, a(4*n + 1) = a(n), a(8*n) = A080995(n). - Michael Somos, Jan 28 2011 a(n) = (-1)^n * A089802(n). For n > 0, a(n) = b(n)-b(n-1) + c(n)-c(n-1), where b(n) = floor(sqrt(n/3+1/9)+2/3) and c(n) = floor(sqrt(n/3+1/9)+4/3). - Mikael Aaltonen, Jan 22 2015 a(n) = A033684(3*n + 1). - Michael Somos, Jan 10 2017 EXAMPLE G.f. = 1 + x + x^5 + x^8 + x^16 + x^21 + x^33 + x^40 + x^56 + x^65 + x^85 + ... G.f. = q + q^4 + q^16 + q^25 + q^49 + q^64 + q^100 + q^121 + q^169 + q^196 + ... MAPLE A089801 := proc(n)         A033684(3*n+1) ; end proc: # R. J. Mathar, Oct 07 2011 M:=33; S:=f->series(f, q, 500); L:=f->seriestolist(f); X:=add(q^(3*n^2+2*n), n=-M..M); S(%); L(%); # N. J. A. Sloane, Jan 31 2012 eps:=Array(0..120, 0); for j from 0 to 120 do if 3*j^2+2*j <= 120 then eps[3*j^2+2*j] := 1; fi; if 3*j^2+4*j+1 <= 120 then eps[3*j^2+4*j+1] := 1; fi; end do;  # N. J. A. Sloane, Aug 12 2017 MATHEMATICA a[ n_] := SeriesCoefficient[ (1/2) x^(-1/3) (EllipticTheta[ 3, 0, x^(1/3)] - EllipticTheta[ 3, 0, x^3]), {x, 0, n}]; (* Michael Somos, Jun 29 2012 *) a[ n_] := SeriesCoefficient[ 2^(-1/2) x^(-3/8) QPochhammer[ -x, x^2] EllipticTheta[ 2, Pi/4, x^(3/2)], {x, 0, n}]; (* Michael Somos, Jun 29 2012 *) PROG (PARI) {a(n) = issquare(3*n + 1)}; /* Michael Somos, Apr 12 2005 */ (MAGMA) Basis( ModularForms( Gamma0(36), 1/2), 87) ; /* Michael Somos, Jul 02 2014 */ CROSSREFS Cf. A001082, A033684, A080995, A089802, A089807, A280739. Sequence in context: A185059 A179776 A089802 * A290739 A143064 A185124 Adjacent sequences:  A089798 A089799 A089800 * A089802 A089803 A089804 KEYWORD nonn AUTHOR Eric W. Weisstein, Nov 12 2003 EXTENSIONS Edited with simpler definition by N. J. A. Sloane, Jan 31 2012 Further edited by N. J. A. Sloane, Aug 12 2017 STATUS approved

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Last modified July 2 14:41 EDT 2020. Contains 335401 sequences. (Running on oeis4.)