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A097057
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Number of integer solutions to a^2 + b^2 + 2*c^2 + 2*d^2 = n.
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20
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1, 4, 8, 16, 24, 24, 32, 32, 24, 52, 48, 48, 96, 56, 64, 96, 24, 72, 104, 80, 144, 128, 96, 96, 96, 124, 112, 160, 192, 120, 192, 128, 24, 192, 144, 192, 312, 152, 160, 224, 144, 168, 256, 176, 288, 312, 192, 192, 96, 228, 248, 288, 336, 216, 320, 288, 192, 320, 240, 240
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OFFSET
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0,2
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COMMENTS
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a^2 + b^2 + 2*c^2 + 2*d^2 is another (cf. A000118) of Ramanujan's 54 universal quaternary quadratic forms. - Michael Somos, Apr 01 2008
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REFERENCES
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B. C. Berndt, Ramanujan's Notebooks Part V, Springer-Verlag, see p. 373 Entry 31.
Jesse Ira Deutsch, Bumby's technique and a result of Liouville on a quadratic form, Integers 8 (2008), no. 2, A2, 20 pp. MR2438287 (2009g:11047).
N. J. Fine, Basic Hypergeometric Series and Applications, Amer. Math. Soc., 1988; p. 78, Eq. (32.29).
S. Ramanujan, Collected Papers, Chap. 20, Cambridge Univ. Press 1927 (Proceedings of the Camb. Phil. Soc., 19 (1917), 11-21).
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LINKS
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FORMULA
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Euler transform of period 8 sequence [4, -2, 4, -8, 4, -2, 4, -4, ...]. - Michael Somos, Sep 17 2004
Multiplicative with a(n) = 4*b(n), b(2) = 2, b(2^e) = 6 if e > 1, b(p^e) = (p^(e+1) - 1) / (p - 1) if p > 2. - Michael Somos, Sep 17 2004
Expansion of (eta(q^2) * eta(q^4))^6 / (eta(q) * eta(q^8))^4 in powers of q.
G.f. is a period 1 Fourier series which satisfies f(-1 / (8 t)) = 8 (t/i)^2 f(t) where q = exp(2 Pi i t). - Michael Somos, Jul 05 2011
G.f.: (theta_3(q) * theta_3(q^2))^2.
G.f.: Product_{k>0} ((1-x^(2k))(1-x^(4k)))^6/((1-x^k)(1-x^(8k)))^4.
G.f.: 1 + Sum_{k>0} 8 * x^(4*k) / (1 + x^(4*k))^2 + 4 * x^(2*k-1) / (1 - x^(2*k-1))^2 = 1 + Sum_{k>0} (2 + (-1)^k) * 4*k * x^(2*k) / (1 + x^(2*k)) + 4*(2*k - 1) * x^(2*k-1) / (1 - x^(2*k - 1)). - Michael Somos, Oct 22 2005
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EXAMPLE
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1 + 4*q + 8*q^2 + 16*q^3 + 24*q^4 + 24*q^5 + 32*q^6 + 32*q^7 + 24*q^8 + ...
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MATHEMATICA
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a[ n_] := SeriesCoefficient[ (EllipticTheta[ 3, 0, q] EllipticTheta[ 3, 0, q^2])^2, {q, 0, n}] (* Michael Somos, Jul 05 2011 *)
f[p_, e_] := (p^(e+1)-1)/(p-1); f[2, 1] = 2; f[2, e_] := 6; a[0] = 1; a[1] = 4; a[n_] := 4 * Times @@ f @@@ FactorInteger[n]; Array[a, 100, 0] (* Amiram Eldar, Aug 22 2023 *)
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PROG
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(PARI) {a(n) = local(t); if( n<1, n>=0, t = 2^valuation( n, 2); 4 * sigma(n/t) * if( t>2, 6, t))} \\ Michael Somos, Sep 17 2004
(PARI) {a(n) = local(A = x * O(x^n)); polcoeff( (eta(x^2 + A) * eta(x^4 + A))^6 / (eta(x + A) * eta(x^8 + A))^4, n))} \\ Michael Somos, Sep 17 2004
(PARI) {a(n) = if( n<1, n==0, 2 * qfrep([ 1, 0, 0, 0; 0, 1, 0, 0; 0, 0, 2, 0; 0, 0, 0, 2], n)[n])} \\ Michael Somos, Oct 29 2005
(PARI) A097057(n)=if(n, sigma(n>>n=valuation(n, 2))*if(n>1, 24, 4<<n), 1) \\ M. F. Hasler, May 07 2018
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CROSSREFS
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a^2 + b^2 + 2*c^2 + m*d^2 = n: this sequence (m=2), A320124 (m=3), A320125 (m=4), A320126 (m=5), A320127 (m=6), A320128 (m=7), A320130 (m=8), A320131 (m=9), A320132 (m=10), A320133 (m=11), A320134 (m=12), A320135 (m=13), A320136 (m=14).
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KEYWORD
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nonn,mult
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AUTHOR
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EXTENSIONS
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Added keyword mult and minor edits by M. F. Hasler, May 07 2018
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STATUS
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approved
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