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A209456 a(n) = Pell(n)*A028594(n) for n>=1, with a(0)=1, where A028594 lists the coefficients in (theta_3(x)*theta_3(7*x)+theta_2(x)*theta_2(7*x))^2. 3
1, 4, 24, 80, 336, 696, 3360, 676, 24480, 51220, 171216, 275568, 1552320, 1873816, 969384, 18722400, 58383168, 81841608, 428096760, 530008720, 2687063904, 617823440, 13424019552, 21605633376, 130401532800, 162655527004, 532025081616, 1223259207200 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Compare g.f. to the Lambert series of A028594:

1 + 4*Sum_{n>=1} Chi(n,7)*n*x^n/(1-x^n).

Here Chi(n,7) = principal Dirichlet character of n modulo 7.

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..1000

FORMULA

G.f.: 1 + 4*Sum_{n>=1} Pell(n)*Chi(n,7)*n*x^n/(1 - A002203(n)*x^n + (-1)^n*x^(2*n)), where A002203(n) = Pell(n-1) + Pell(n+1).

EXAMPLE

G.f.: A(x) = 1 + 4*x + 24*x^2 + 80*x^3 + 336*x^4 + 696*x^5 + 3360*x^6 +...

where A(x) = 1 + 1*4*x + 2*12*x^2 + 5*16*x^3 + 12*28*x^4 + 29*24*x^5 + 70*48*x^6 + 169*4*x^7 + 408*60*x^8 + 985*52*x^9 +...+ Pell(n)*A028594(n)*x^n +...

The g.f. is also given by the identity:

A(x) = 1 + 4*( 1*1*x/(1-2*x-x^2) + 2*2*x^2/(1-6*x^2+x^4) + 5*3*x^3/(1-14*x^3-x^6) + 12*4*x^4/(1-34*x^4+x^8) + 29*5*x^5/(1-82*x^5-x^10) + 70*6*x^6/(1-198*x^6+x^12) + 0*169*7*x^7/(1+478*x^7-x^14) +...).

The values of the Dirichlet character Chi(n,7) repeat [1,1,1,1,1,1,0, ...].

MATHEMATICA

A028594[n_]:= If[n < 1, Boole[n == 0], 4*Sum[If[Mod[d, 7] > 0, d, 0], {d, Divisors@n}]]; Join[{1}, Table[Fibonacci[n, 2]*A028594[n], {n, 1, 50}]] (* G. C. Greubel, Jan 03 2018 *)

PROG

(PARI) {Pell(n)=polcoeff(x/(1-2*x-x^2+x*O(x^n)), n)}

{A002203(n)=Pell(n-1)+Pell(n+1)}

{a(n)=polcoeff(1 + 4*sum(m=1, n, Pell(m)*kronecker(m, 7)^2*m*x^m/(1-A002203(m)*x^m+(-1)^m*x^(2*m) +x*O(x^n))), n)}

for(n=0, 35, print1(a(n), ", "))

CROSSREFS

Cf. A028594, A205976, A209455, A204270, A000129 (Pell), A002203.

Sequence in context: A025220 A112742 A158494 * A069145 A264184 A211071

Adjacent sequences:  A209453 A209454 A209455 * A209457 A209458 A209459

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Mar 10 2012

STATUS

approved

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Last modified July 27 02:39 EDT 2021. Contains 346302 sequences. (Running on oeis4.)