OFFSET
0,2
COMMENTS
The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice.
Convolution square of A004016.
Denoted by E_{2,3}^{i\infinity}(\tau) in Kaneko and Sakai 2012 on page 7. - Michael Somos, Dec 27 2014
REFERENCES
Bruce C. Berndt, Ramanujan's Notebooks Part III, Springer-Verlag, 1991, see p. 460, Entry 3(i).
J. H. Conway and N. J. A. Sloane, Sphere Packings, Lattices and Groups, Springer-Verlag, 1999, p. 110.
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..10000
Michael Gilleland, Some Self-Similar Integer Sequences.
Masanobu Kaneko and Yuichi Sakai, The Ramanujan-Serre Differential Operators and certain Elliptic Curves, arXiv:1201.1685 [math.NT], 2012.
Masao Koike, Modular forms on non-compact arithmetic triangle groups, Unpublished manuscript [Extensively annotated with OEIS A-numbers by N. J. A. Sloane, Feb 14 2021. I wrote 2005 on the first page but the internal evidence suggests 1997.]
Gabriele Nebe and N. J. A. Sloane, Home page for hexagonal (or triangular) lattice A2.
FORMULA
Expansion of (theta_3(z)*theta_3(3z)+theta_2(z)*theta_2(3z))^2.
Expansion of a(q)^2 in powers of q where a() is a cubic AGM theta function.
G.f. A(x) satisfies 0 = f(A(x), A(x^2), A(x^4)) where f(u, v, w) = u^2 + 9*v^2 + 16*w^2 - 6*u*v + 4*u*w - 24*v*w. - Michael Somos, Jul 19 2004
G.f.: 1 + 12* Sum_{k>0} x^k / (1 - x^k)^2 - 36* Sum_{k>0} x^(3*k) / (1 - x^(3*k))^2. - Michael Somos, Apr 15 2007
a(n) = 12 * A046913(n) unless n=0.
Sum_{k=1..n} a(k) ~ c * n^2, where c = 2*Pi^2/3. - Amiram Eldar, Jan 21 2024
EXAMPLE
G.f. = 1 + 12*q + 36*q^2 + 12*q^3 + 84*q^4 + 72*q^5 + 36*q^6 + 96*q^7 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ ((QPochhammer[ q]^3 + 9 q QPochhammer[ q^9]^3) / QPochhammer[ q^3])^2, {q, 0, n}]; (* Michael Somos, May 26 2014 *)
a[ n_] := If[ n < 1, Boole[ n == 0], 12 Sum[ If[ Mod[ d, 3] > 0, d, 0], {d, Divisors @ n }]]; (* Michael Somos, May 26 2014 *)
PROG
(PARI) {a(n) = if( n<1, n==0, 12 * (sigma(3*n) - 3*sigma(n)))}; /* Michael Somos, Jul 19 2004 */
(PARI) {a(n) = if( n<0, 0, polcoeff( sum(k=1, n, 6 * x^k / (1 + x^k + x^(2*k)), 1 + x * O(x^n))^2, n))}; /* Michael Somos, Jul 19 2004 */
(Sage) ModularForms( Gamma0(3), 2, prec=70).0; # Michael Somos, Jun 12 2014
(Magma) Basis( ModularForms( Gamma0(3), 2), 70)[1]; /* Michael Somos, Jun 12 2014 */
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved