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 A008457 a(n) = Sum_{ d >= 1, d divides n} (-1)^(n-d)*d^3. 12
 1, 7, 28, 71, 126, 196, 344, 583, 757, 882, 1332, 1988, 2198, 2408, 3528, 4679, 4914, 5299, 6860, 8946, 9632, 9324, 12168, 16324, 15751, 15386, 20440, 24424, 24390, 24696, 29792, 37447, 37296, 34398, 43344, 53747, 50654, 48020, 61544, 73458 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The modular form (e(1)-e(2))(e(1)-e(3)) for GAMMA_0 (2) (with constant term -1/16 omitted). a(n) = r_8(n)/16, where r_8(n) = A000143(n) is the number of integral solutions of Sum_{j=1..8} x_j^2 = n (with the order of the summands respected). See the Grosswald reference, and the Hardy reference, pp. 146-147, eq. (9.9.3) and sect. 9.10. - Wolfdieter Lang, Jan 09 2017 REFERENCES N. J. Fine, Basic Hypergeometric Series and Applications, Amer. Math. Soc., 1988; p. 77, Eq. (31.6). E. Grosswald, Representations of Integers as Sums of Squares. Springer-Verlag, NY, 1985, p. 121, eq. (9.19). G. H. Hardy, Ramanujan: twelve lectures on subjects suggested by his life and work, AMS Chelsea Publishing, Providence, Rhode Island, 2002, p. 142. F. Hirzebruch et al., Manifolds and Modular Forms, Vieweg 1994 pp. 77, 133. H. Petersson, Modulfunktionen und Quadratische Formen, Springer-Verlag, 1982; p. 179. LINKS G. C. Greubel, Table of n, a(n) for n = 1..5000 M. Peters, Sums of nine squares, Acta Arith., 102 (2002), 131-135. FORMULA Multiplicative with a(2^e) = (8^(e+1)-15)/7, a(p^e) = (p^(3*e+3)-1)/(p^3-1), p > 2. - Vladeta Jovovic, Sep 10 2001 a(n) = (-1)^n*(sum of cubes of even divisors of n - sum of cubes of odd divisors of n). Sum_{n>0} n^3*x^n*(15*x^n-(-1)^n)/(1-x^(2*n)). - Vladeta Jovovic, Oct 24 2002 G.f.: Sum_{k>0} k^3 x^k/(1 - (-x)^k). - Michael Somos, Sep 25 2005 G.f.: (1/16)*(-1+(Product_{k>0} (1-(-q)^k)/(1+(-q)^k))^8). [corrected by Vaclav Kotesovec, Sep 26 2015] Dirichlet g.f. zeta(s)*zeta(s-3)*(1-2^(1-s)+2^(4-2s)), Dirichlet convolution of A001158 and the quasi-finite (1,-2,0,16,0,0,...). - R. J. Mathar, Mar 04 2011 A138503(n) = -(-1)^n * a(n). Bisection: a(2*k-1) = A001158(2*k-1), a(2*k) = 8*A001158(k) - A051000(k), k >= 1. In the Hardy reference a(n) = sigma^*_3(n). - Wolfdieter Lang, Jan 07 2017 G.f.: (theta_3(x)^8 - 1)/16, where theta_3() is the Jacobi theta function. - Ilya Gutkovskiy, Apr 17 2018 EXAMPLE G.f. = q + 7*q^2 + 28*q^3 + 71*q^4 + 126*q^5 + 196*q^6 + 344*q^7 + 583*q^8 + ... MAPLE (1/16)*product((1+q^n)^8/(1-q^n)^8, n=1..60); MATHEMATICA nmax = 40; Rest[CoefficientList[Series[Product[((1-(-q)^k)/(1+(-q)^k))^8, {k, 1, nmax}]/16, {q, 0, nmax}], q]] (* Vaclav Kotesovec, Sep 26 2015 *) a[n_] := DivisorSum[n, (-1)^(n-#)*#^3&]; Array[a, 40] (* Jean-François Alcover, Dec 01 2015 *) a[ n_] := SeriesCoefficient[ (EllipticTheta[ 3, 0, x]^8 - 1) / 16, {x, 0, n}]; (* Michael Somos, Aug 10 2018 *) PROG (PARI) {a(n) = if( n<1, 0, (-1)^n * sumdiv(n, d, (-1)^d * d^3))}; /* Michael Somos, Sep 25 2005 */ CROSSREFS Cf. A000143, A001158, A051000, A064027, A002129, A048272, A138503. Sequence in context: A230285 A033582 A176362 * A138503 A223765 A064951 Adjacent sequences:  A008454 A008455 A008456 * A008458 A008459 A008460 KEYWORD nonn,mult AUTHOR STATUS approved

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Last modified May 24 00:38 EDT 2019. Contains 323528 sequences. (Running on oeis4.)