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 A005928 G.f.: s(1)^3/s(3), where s(k) = eta(q^k) and eta(q) is Dedekind's function, cf. A010815. (Formerly M2202) 200
 1, -3, 0, 6, -3, 0, 0, -6, 0, 6, 0, 0, 6, -6, 0, 0, -3, 0, 0, -6, 0, 12, 0, 0, 0, -3, 0, 6, -6, 0, 0, -6, 0, 0, 0, 0, 6, -6, 0, 12, 0, 0, 0, -6, 0, 0, 0, 0, 6, -9, 0, 0, -6, 0, 0, 0, 0, 12, 0, 0, 0, -6, 0, 12, -3, 0, 0, -6, 0, 0, 0, 0, 0, -6, 0, 6, -6, 0, 0, -6, 0, 6, 0, 0, 12, 0, 0, 0, 0, 0, 0, -12, 0, 12, 0, 0, 0, -6, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Unsigned sequence is expansion of theta series of hexagonal net with respect to a node. Cubic AGM theta functions: a(q) (see A004016), b(q) (this: A005928), c(q) (A005882). Denoted by a_3(n) in Kassel and Reutenauer 2015. - Michael Somos, Jun 04 2015 REFERENCES N. J. Fine, Basic Hypergeometric Series and Applications, Amer. Math. Soc., 1988; p. 79, Eq. (32.34). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Seiichi Manyama, Table of n, a(n) for n = 0..10000 (terms 0..1000 from T. D. Noe) J. M. Borwein and P. B. Borwein, A cubic counterpart of Jacobi's identity and the AGM, Trans. Amer. Math. Soc., 323 (1991), no. 2, 691-701. MR1010408 (91e:33012) see page 695. J. M. Borwein, P. B. Borwein and F. Garvan, Some Cubic Modular Identities of Ramanujan, Trans. Amer. Math. Soc. 343 (1994), 35-47. Christian Kassel and Christophe Reutenauer, The zeta function of the Hilbert scheme of n points on a two-dimensional torus, arXiv:1505.07229v3 [math.AG], 2015. [A later version of this paper has a different title and different contents, and the number-theoretical part of the paper was moved to the publication below.] Christian Kassel and Christophe Reutenauer, Complete determination of the zeta function of the Hilbert scheme of n points on a two-dimensional torus, arXiv:1610.07793 [math.NT], 2016. N. J. A. Sloane, Theta series and magic numbers for diamond and certain ionic crystal structures, J. Math. Phys. 28 (1987), 1653-1657. FORMULA a(n) is the coefficient of q^n in b(q)=eta(q)^3/eta(q^3) = (3/2)*a(q^3)-a(q)/2 where a(q)=theta(Hexagonal) - Kok Seng Chua (chuaks(AT)ihpc.nus.edu.sg), May 07 2002 From Michael Somos, May 20 2005: (Start) Euler transform of period 3 sequence [ -3, -3, -2, ...]. a(n) = -3 * b(n) except for a(0) = 1, where b() is multiplicative with b(p^e) = -2 if p = 3 and e>0, b(p^e) = e+1 if p == 1 (mod 6), b(p^e) = (1 + (-1)^e)/2 if p == 2, 5 (mod 6). G.f. A(x) satisfies 0 = f(A(x), A(x^2), A(x^4)) where f(u, v, w) = v^3 - 2*u*w^2 + u^2*w. G.f. A(x) satisfies 0 = f(A(x), A(x^2), A(x^3), A(x^6)) where f(u1, u2, u3, u6) = u1^2*u6 - 2*u1*u2*u6 + 4*u2^2*u6 - 3*u2*u3^2. G.f. A(x) satisfies 0 = f(A(x), A(x^2), A(x^3), A(x^6)) where f(u1, u2, u3, u6) = u1*u2*u3 + u1^2*u3 - 3*u1*u6^2 + u2^2*u3. (End) a(3*n + 2) = 0. a(3*n + 1) = -A005882(n), a(3*n) = A004016(n). - Michael Somos, Jul 15 2005 a(n) = -3 * A123477(n) unless n=0. |a(n)| = A113062(n). Moebius transform is period 9 sequence [-3, 3, 9, -3, 3, -9, -3, 3, 0, ...]. - Michael Somos, Dec 25 2007 Expansion of b(q) = a(q^3) - c(q^3) in powers of q where a(), b(), c() are cubic AGM theta functions. - Michael Somos, Dec 25 2007 G.f. is a period 1 Fourier series which satisfies f(-1 / (3 t)) = 3^(3/2) (t/i) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A033687. G.f.: exp( sum(n>=1, (sigma(n)-sigma(3*n))*x^n/n ) ). - Joerg Arndt, Jul 30 2011 a(n) = (-1)^(mod(n, 3) = 1) * A113062(n). - Michael Somos, Sep 05 2014 a(2*n + 1) = -3 * A123530(n). a(4*n) = a(n). a(4*n + 1) = -3 * A253243(n). a(4*n + 2) = 0. a(4*n + 3) = 6 * A246838(n). a(6*n + 1) = -3 * A097195(n). a(6*n + 3) = 6 * A033762(n). - Michael Somos, Jun 04 2015 G.f.: 1 + Sum_{k>0} -3 * x^k / (1 + x^k + x^(2*k)) + 9 * x^(3*k) / (1 + x^(3*k) + x^(6*k)). - Michael Somos, Jun 04 2015 a(0) = 1, a(n) = -(3/n)*Sum_{k=1..n} A078708(k)*a(n-k) for n > 0. - Seiichi Manyama, Apr 29 2017 EXAMPLE G.f. = 1 - 3*q + 6*q^3 - 3*q^4 - 6*q^7 + 6*q^9 + 6*q^12 - 6*q^13 - 3*q^16 + ... MATHEMATICA a[ n_] := SeriesCoefficient[ QPochhammer[ q]^3 / QPochhammer[ q^3], {q, 0, n}]; (* Michael Somos, May 24 2013 *) a[ n_] := If[ n < 1, Boole[ n==0], -3 Sum[{1, -1, -3, 1, -1, 3, 1, -1, 0}[[ Mod[ d, 9, 1]]], {d, Divisors @ n}]]; (* Michael Somos, Sep 23 2013 *) PROG (PARI) {a(n) = my(A, p, e); if( n<1, n==0, A = factor(n); -3 * prod( k=1, matsize(A)[1], [p, e] = A[k, ]; if( p==3, -2, if( p%6==1, e+1, !(e%2)))))}; \\ Michael Somos, May 20 2005 (PARI) {a(n) = my(A = x * O(x^n)); polcoeff( eta(x + A)^3 / eta(x^3 + A), n)}; \\ Michael Somos, May 20 2005 (PARI) {a(n) = if( n<1, n==0, sumdiv(n, d, [0, -3, 3, 9, -3, 3, -9, -3, 3] [d%9 + 1]))}; \\ Michael Somos, Dec 25 2007 (PARI) N=66; x='x+O('x^N); gf=exp(sum(n=1, N, (sigma(n)-sigma(3*n))*x^n/n)); Vec(gf) \\ Joerg Arndt, Jul 30 2011 (PARI) lista(nn) = {q='q+O('q^nn); Vec(eta(q)^3/eta(q^3))} \\ Altug Alkan, Mar 20 2018 (MAGMA) A := Basis( ModularForms( Gamma1(9), 1), 100); A[1] - 3*A[2] + 6*A[4]; \\ Michael Somos, Jan 31 2015 CROSSREFS Cf. A005882, A033762, A097195, A113062, A123477, A123530, A246838, A253243. Sequence in context: A011079 A226535 * A113062 A259659 A005889 A095712 Adjacent sequences:  A005925 A005926 A005927 * A005929 A005930 A005931 KEYWORD sign AUTHOR EXTENSIONS Edited by M. F. Hasler, May 07 2018 STATUS approved

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Last modified August 16 16:40 EDT 2018. Contains 313809 sequences. (Running on oeis4.)