1,3

Number of integer solutions to the equation x^2 + 2*y^2 = n when (-x, -y) and (x, y) are counted as the same solution.

For n nonzero, a(n) is nonzero if and only if n is in A002479. - Michael Somos, Dec 15 2011

B. C. Berndt, Ramanujan's Notebooks Part III, Springer-Verlag, see p. 114 Entry 8(iii).

L. E. Dickson, History of the Theory of Numbers. Carnegie Institute Public. 256, Washington, DC, Vol. 1, 1919; Vol. 2, 1920; Vol. 3, 1923, see vol. 3, p. 19.

N. J. Fine, Basic Hypergeometric Series and Applications, Amer. Math. Soc., 1988; p. 78, Eq. (32.24).

J. W. L. Glaisher, Table of the excess of the number of (8k+1)- and (8k+3)-divisors of a number over the number of (8k+5)- and (8k+7)-divisors, Messenger Math., 31 (1901), 82-91.

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).

T. D. Noe, Table of n, a(n) for n = 1..10000

J. W. L. Glaisher, Table of the excess of the number of (8k+1)- and (8k+3)-divisors of a number over the number of (8k+5)- and (8k+7)-divisors, Messenger Math., 31 (1901), 82-91. [Incomplete annotated scanned copy]

M. D. Hirschhorn, The number of representations of a number by various forms, Discrete Mathematics 298 (2005), 205-211

N. J. A. Sloane, Transforms

Index entries for sequences related to Glaisher's numbers

Coefficients in expansion of Dirichlet series Product_p (1-(Kronecker(m, p)+1)*p^(-s)+Kronecker(m, p)*p^(-2s))^(-1) for m = -2.

Moebius transform is period 8 sequence [ 1, 0, 1, 0, -1, 0, -1, 0, ...]. - Michael Somos, Aug 23 2005

G.f.: (theta_3(q) * theta_3(q^2) - 1) / 2 = Sum_{k>0} kronecker( -2, n) * x^k / (1 - x^k) = Sum_{k>0} (x^k + x^(3*k)) / (1 + x^(4*k)).

Multiplicative with a(2^e) = 1, a(p^e) = e+1 if p == 1, 3 (mod 8), a(p^e) = (1+(-1)^e)/2 if p == 5, 7 (mod 8). - Michael Somos, Oct 23 2006

A033715(n) = 2 * a(n) unless n=0.

a(n) = A188169(n)+A188170(n)-A188171(n)-A188172(n) [Hirschhorn] - R. J. Mathar, Mar 23 2011

G.f.: A(x)=2*(1+x^2)/(G(0)-2*x*(1+x^2)) ; G(k)=1+x+x^(2*k)*(1+x^3+x^(2*k+1)+x^(2*k+4)+x^(4*k+3)+x^(4*k+4))-x*(1+x^(2*k))*(1+x^(2*k+4))*(1+x^(4*k+4))^2/G(k+1); (continued fraction Euler's kind,1-step ).- Sergei N. Gladkovskii, Jan 03 2012

x + x^2 + 2*x^3 + x^4 + 2*x^6 + x^8 + 3*x^9 + 2*x^11 + 2*x^12 + x^16 + ...

a[n_] := Total[ KroneckerSymbol[-8, #] & /@ Divisors[n]]; Table[a[n], {n, 1, 105}] (* Jean-François Alcover, Nov 25 2011, after Michael Somos *)

(PARI) a(n) = if( n<1, 0, issquare(n)-issquare(2*n) + 2*sum(i=1, sqrtint(n\2), issquare(n-2*i^2)))

(PARI) {a(n) = if( n<1, 0, qfrep([ 1, 0; 0, 2], n)[n])} /* Michael Somos, Jun 05 2005 */

(PARI) {a(n) = if( n<1, 0, direuler(p=2, n, 1 / (1 - X) / (1 - kronecker( -2, p) * X))[n])} /* Michael Somos, Jun 05 2005 */

(PARI) {a(n) = if( n<1, 0, sumdiv(n, d, kronecker( -2, d)))} /* Michael Somos, Aug 23 2005 */

(PARI) {a(n) = local(A, p, e); if( n<1, 0, A = factor(n); prod( k=1, matsize(A)[1], if( p=A[k, 1], e=A[k, 2]; if( p==2, 1, if( p%8<4, e+1, !(e%2))))))} /* Michael Somos, Oct 23 2006 */

(PARI) {a(n) = local(A); if( n<1, 0, A = x * O(x^n); polcoeff( eta(x + A)^-2 * eta(x^2 + A)^3 * eta(x^4 + A)^3 * eta(x^8 + A)^-2, n) / 2)}

(PARI) a(n) = my(f=factor(n>>valuation(n, 2)), e); prod(i=1, #f~, e=f[i, 2]; if( f[i, 1]%8<4, e+1, 1 - e%2)) \\ Charles R Greathouse IV, Sep 09 2014

Cf. A033715.

Sequence in context: A220136 * A129134 A133693 A065675 A244553 A231167

Adjacent sequences:  A002322 A002323 A002324 * A002326 A002327 A002328

nonn,easy,nice,mult

N. J. A. Sloane.

approved