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 A050456 a(n) = Sum_{d|n, d==1 mod 4} d^4 - Sum_{d|n, d==3 mod 4} d^4. 6
 1, 1, -80, 1, 626, -80, -2400, 1, 6481, 626, -14640, -80, 28562, -2400, -50080, 1, 83522, 6481, -130320, 626, 192000, -14640, -279840, -80, 391251, 28562, -524960, -2400, 707282, -50080, -923520, 1, 1171200, 83522, -1502400, 6481, 1874162, -130320 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Multiplicative because it is the Inverse Möbius transform of [1 0 -3^4 0 5^4 0 -7^4 ...], which is multiplicative. - Christian G. Bower, May 18 2005 Called E_4(n) by Hardy. Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700). REFERENCES E. Grosswald, Representations of Integers as Sums of Squares. Springer-Verlag, NY, 1985, p. 120. G. H. Hardy, Ramanujan: twelve lectures on subjects suggested by his life and work, Chelsea Publishing Company, New York 1959, p. 135 section 9.3. MR0106147 (21 #4881) LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 J. W. L. Glaisher, On the representations of a number as the sum of two, four, six, eight, ten, and twelve squares, Quart. J. Math. 38 (1907), 1-62 (see p. 4 and p. 8). K. Ono, S. Robins and P. T. Wahl, On the representation of integers as sums of triangular numbers, Aequationes mathematicae, August 1995, Volume 50, Issue 1-2, pp 73-94. Case k=10. Michael Somos, Introduction to Ramanujan theta functions Eric Weisstein's World of Mathematics, Ramanujan Theta Functions FORMULA Expansion of ( eta(q)^4 * eta(q^2)^2 * (5 * eta(q)^8 / eta(q^4)^4 + 64 * q * eta(q^4)^4 ) - 5) / 4 in powers of q. - Michael Somos, Jan 14 2012 Expansion of (phi(x)^2 * (5 * phi(-x)^8 + 64 * x * psi(-x)^8) - 5) / 4 in powers of x where phi(), psi() are Ramanujan theta functions. - Michael Somos, Jan 14 2012 a(n) is multiplicative with a(2^e) = 1, a(p^e) = ((p^4)^(e+1) - 1) / (p^4 - 1) if p == 1 (mod 4), a(p^e) = (1 - (-p^4)^(e+1)) / (1 + p^4) if p == 3 (mod 4). - Michael Somos, Jan 14 2012 G.f.: Sum_{k>0} (-1)^(k-1) * (2*k - 1)^4 * x^(2*k - 1) / (1 - x^(2*k - 1)). a(n) = Sum_{d|n} Chi(d)*d^4, with Chi(n) the second Dirichlet character modulo 4, i.e., Chi(1) = 1 and Chi(3) = -1. See the Ono et al ref. case k=10. - Wolfdieter Lang, Jan 13 2017 EXAMPLE G.f. = x + x^2 - 80*x^3 + x^4 + 626*x^5 - 80*x^6 - 2400*x^7 + x^8 + 6481*x^9 + ... MATHEMATICA e[r_, n_] := Plus@@(Select[Divisors[n], Mod[#, 4] == 1 &]^r) -Plus@@(Select[Divisors[n], Mod[#, 4] == 3 &]^r); e[4, #] &/@Range[38] (* Ant King, Nov 10 2012 *) PROG (PARI) {a(n) = if( n<1, 0, sumdiv( n, d, (d%2) * (-1)^((d-1)/2) * d^4))}; /* Michael Somos, Sep 12 2005 */ (PARI) {a(n) = if( n<1, 0, sumdiv( n, d, d^4 * kronecker( -4, d)))}; (PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( ( eta(x + A)^4 * eta(x^2 + A)^2 * (5 * eta(x + A)^8 / eta(x^4 + A)^4 + 64 * x * eta(x^4 + A)^4 ) - 5) / 4, n))}; /* Michael Somos, Jan 14 2012 */ CROSSREFS Sequence in context: A093404 A236236 A031136 * A107930 A239131 A033400 Adjacent sequences: A050453 A050454 A050455 * A050457 A050458 A050459 KEYWORD sign,mult AUTHOR N. J. A. Sloane, Dec 23 1999 STATUS approved

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Last modified December 8 07:38 EST 2022. Contains 358691 sequences. (Running on oeis4.)