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 A002324 Number of divisors of n == 1 (mod 3) minus number of divisors of n == 2 (mod 3). (Formerly M0016 N0002) 36
 1, 0, 1, 1, 0, 0, 2, 0, 1, 0, 0, 1, 2, 0, 0, 1, 0, 0, 2, 0, 2, 0, 0, 0, 1, 0, 1, 2, 0, 0, 2, 0, 0, 0, 0, 1, 2, 0, 2, 0, 0, 0, 2, 0, 0, 0, 0, 1, 3, 0, 0, 2, 0, 0, 0, 0, 2, 0, 0, 0, 2, 0, 2, 1, 0, 0, 2, 0, 0, 0, 0, 0, 2, 0, 1, 2, 0, 0, 2, 0, 1, 0, 0, 2, 0, 0, 0, 0, 0, 0, 4, 0, 2, 0, 0, 0, 2, 0, 0, 1, 0, 0, 2, 0, 0, 0, 0, 1, 2, 0, 2, 2, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,7 COMMENTS Coefficients in expansion of Dirichlet series Product_p (1 - (Kronecker(m,p) + 1)*p^(-s) + Kronecker(m,p) * p^(-2s))^(-1) for m = -3. (Number of points of norm n in hexagonal lattice) / 6, n>0. The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice. REFERENCES J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, p. 112, first display. J. W. L. Glaisher, Table of the excess of the number of (3k+1)-divisors of a number over the number of (3k+2)-divisors, Messenger Math., 31 (1901), 64-72. D. H. Lehmer, Guide to Tables in the Theory of Numbers. Bulletin No. 105, National Research Council, Washington, DC, 1941, pp. 7-10. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 G. E. Andrews, Three aspects of partitions, Séminaire Lotharingien de Combinatoire, B25f (1990), 1 p. H. M. Farkas, On an arithmetical function, Ramanujan J., 8(3) (2004), 309-315. Pavel Guerzhoy, Ka Lun Wong, Farkas' identities with quartic characters, arXiv:1905.06506 [math.NT], 2019. 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. G. Nebe and N. J. A. Sloane, Home page for hexagonal (or triangular) lattice A2 José Manuel Rodríguez Caballero, Divisors on overlapped intervals and multiplicative functions, arXiv:1709.09621 [math.NT], 2017. J. S. Rutherford, Generating functions for the cage isomers of the C_{20n} icosahedral fullerenes, J. Mathematical Chem., 14 (1993), 385-390. [From N. J. A. Sloane, Mar 12 2009] John S. Rutherford, Sublattice enumeration. IV. Equivalence classes of plane sublattices by parent Patterson symmetry and colour lattice group type, Acta Cryst. (2009). A65, 156-163. [See Table 1]. - From N. J. A. Sloane, Feb 23 2009 FORMULA G.f. A(x) satisfies 0 = f(A(x), A(x^2), A(x^4)) where f(u, v, w) = u^2 - 3*v^2 + 4*w^2 - 2*u*w + w - v. - Michael Somos, Jul 20 2004 Has a nice Dirichlet series expansion, see PARI line. G.f.: Sum_{k>0} x^k/(1+x^k+x^(2*k)). - Vladeta Jovovic, Dec 16 2002 a(3*n + 2) = 0, a(3*n) = a(n), a(3*n + 1) = A033687(n). - Michael Somos, Apr 04 2003 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 - u3)*(u3 - u6) - (u2 - u6)^2. - Michael Somos, May 20 2005 Multiplicative with a(3^e) = 1, a(p^e) = e+1 if p == 1 (mod 3), a(p^e) = (1+(-1)^e)/2 if p == 2 (mod 3). - Michael Somos, May 20 2005 G.f.: Sum_{k>0} x^(3*k - 2) / (1 - x^(3*k - 2)) - x^(3*k - 1)  / (1 - x^(3*k - 1)). - Michael Somos, Nov 02 2005 G.f.: Sum_{n >= 1} q^(n^2)(1-q)(1-q^2)...(1-q^(n-1))/(((1-q^(n+1))(1-q^(n+2))...(1-q^(2n))). - Jeremy Lovejoy, Jun 12 2009 a(n) = A001817(n) - A001822(n). - R. J. Mathar, Mar 31 2011 A004016(n) = 6*a(n) unless n=0. Dirichlet g.f.: zeta(s)*L(chi_2(3),s), with chi_2(3) the nontrivial Dirichlet character modulo 3 (A102283). - Ralf Stephan, Mar 27 2015 From Andrey Zabolotskiy, May 07 2018: (Start) a(n) = Sum_{ m: m^2|n } A000086(n/m^2). a(A003136(m)) > 0, a(A034020(m)) = 0 for all m. (End) EXAMPLE G.f. = x + x^3 + x^4 + 2*x^7 + x^9 + x^12 + 2*x^13 + x^16 + 2*x^19 + 2*x^21 + ... MAPLE A002324 := proc(n)     A001817(n)-A001822(n) ; end proc: seq(A002324(n), n=1..100) ; # R. J. Mathar, Sep 25 2017 MATHEMATICA dn12[n_]:=Module[{dn=Divisors[n]}, Count[dn, _?(Mod[#, 3]==1&)]-Count[ dn, _?(Mod[#, 3]==2&)]]; dn12/@Range  (* Harvey P. Dale, Apr 26 2011 *) a[ n_] := If[ n < 1, 0, DivisorSum[ n, KroneckerSymbol[ -3, #] &]]; (* Michael Somos, Aug 24 2014 *) Table[DirichletConvolve[DirichletCharacter[3, 2, m], 1, m, n], {n, 1, 30}] (* Steven Foster Clark, May 29 2019 *) PROG (PARI) {a(n) = if( n<0, 0, polcoeff( sum(k=1, n, x^k / (1 + x^k + x^(2*k)), x * O(x^n)), n))}; /* Michael Somos */ (PARI) {a(n) = if( n<1, 0, sumdiv(n, d, (d%3==1) - (d%3==2)))}; (PARI) {a(n) = local(A, p, e); if( n<1, 0, A = factor(n); prod(k=1, matsize(A), if( p=A[k, 1], e=A[k, 2]; if( p==3, 1, if( p%3==1, e+1, !(e%2))))))}; /* Michael Somos, May 20 2005 */ (PARI) {a(n) = if( n<1, 0, qfrep([2, 1; 1, 2], n, 1)[n] / 3)}; /* Michael Somos, Jun 05 2005 */ (PARI) {a(n) = if( n<1, 0, direuler(p=2, n, 1 / (1 - X) / (1 - kronecker(-3, p)*X))[n])}; /* Michael Somos, Jun 05 2005 */ (Haskell) a002324 n = a001817 n - a001822 n  -- Reinhard Zumkeller, Nov 26 2011 CROSSREFS Cf. A004016, A035019, A145377, A293899, A000086, A003136, A034020, A145394. Sequence in context: A117154 A074941 A171774 * A101671 A078979 A063974 Adjacent sequences:  A002321 A002322 A002323 * A002325 A002326 A002327 KEYWORD easy,nonn,nice,mult AUTHOR EXTENSIONS More terms from David Radcliffe Somos d.g.f. replaced with correct version by Ralf Stephan, Mar 27 2015 STATUS approved

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Last modified November 17 05:27 EST 2019. Contains 329217 sequences. (Running on oeis4.)