A104503 Coefficients of the C-Dyson Mod 27 identity.

1, 1, 2, 3, 5, 7, 10, 14, 20, 27, 37, 49, 66, 86, 113, 146, 189, 241, 308, 389, 492, 615, 770, 956, 1187, 1463, 1802, 2207, 2701, 3288, 3999, 4842, 5857, 7056, 8491, 10183, 12197, 14564, 17369, 20658, 24539, 29075, 34408, 40627, 47912, 56385, 66277

Offset: 0

Author

Eric W. Weisstein, Mar 11 2005

Keywords

• nonn

Examples

			1 + q + 2*q^2 + 3*q^3 + 5*q^4 + 7*q^5 + 10*q^6 + 14*q^7 + 20*q^8 + 27*q^9 + ...
		

Links

• G. C. Greubel, Table of n, a(n) for n = 0..1000
• Eric Weisstein's World of Mathematics, Dyson Mod 27 Identities

Crossrefs

Cf. A104501, A104502, A104504.

Programs

Mathematica

QP := QPochhammer; f[x_, y_] := QP[-x, x*y]*QP[-y, x*y]*QP[x*y, x*y]; a[n_]:= SeriesCoefficient[f[-q^12, -q^15]/f[-q, -q^2], {q, 0, n}]; Table[a[n], {n, 0, 50}] (* G. C. Greubel, Apr 08 2018 *)

PARI

{a(n)=local(m); if(n<0, 0, m=sqrtint(24*n+25); polcoeff( sum(k= -((m-5)\18), (m+5)\18, (-1)^k*x^((9*k^2-5*k)*3/2),x*O(x^n))/ eta(x+x*O(x^n)), n))} /* Michael Somos, Mar 15 2006 */

PARI

{a(n)=if(n<1, n==0, polcoeff( sum(k=0, sqrtint(n+1)-1, x^(k^2+2*k)* prod(j=1, k, (1-x^(3*j))/(1-x^j)/(1-x^(2*j+1))/(1-x^(2*j+2)), 1+O(x^(n-k^2-2*k+1)))/(1-x)/(1-x^2) ), n))} /* Michael Somos, Mar 15 2006 */

PARI

{a(n) = local(A); if( n<0, 0, n++; A = eta(x + x*O(x^n)) ; polcoeff( - sum(k=0, n, (k%3==1) * polcoeff(A, k) * x^k) / A, n))} /* Michael Somos, Sep 29 2007 */

Formula

Expansion of f(-q^6,-q^21)/f(-q,-q^2) in powers of q where f() is Ramanujan's theta function.

Given A=A0+A1+A2+A3+A4 is the 5-section, then 0= A0^2*A3^2 +2*A1^2*A2^2 -A0*A2^3 -A3*A1^3 -A0*A1*A2*A3.

G.f.: Product_{k>0} (1-x^(27k))(1-x^(27k-6))(1-x^(27k-21))/(1-x^k).

G.f.: Sum_{k>0} x^(k^2+2k) ( Product_{j=1..k} 1-x^(3j) )/ ( (Product_{j=1..2k+2} (1-x^j)) (Product_{j=1..k}(1-x^j)) ).

A104501(n) = A104503(n-1) + A104504(n-2) unless n=0. - Michael Somos, Sep 29 2007