A259274 G.f.: A(x) = exp( Sum_{n>=1} 4^n * x^n/(n*(1+x^n)) ).

1, 4, 12, 52, 204, 804, 3244, 12948, 51756, 207108, 828364, 3313332, 13253580, 53014116, 212055852, 848224660, 3392897772, 13571588484, 54286358988, 217145432052, 868581718860, 3474326895460, 13897307565804, 55589230225428, 222356920980972, 889427683862724, 3557710735299660

Offset

0,2

Comments

Compare to: exp( Sum_{n>=1} x^n/(1+x^n)/n ) = Sum_{n>=0} x^(n*(n+1)/2).

Links

Table of n, a(n) for n=0..26.

Formula

G.f.: -1/3 + (4/3)/(1+x - 4*x/(1+x^2 - 4*x^2/(1+x^3 - 4*x^3/(1+x^4 - 4*x^4/(1+x^5 - 4*x^5/(1+x^6 - 4*x^6/(1+x^7 - 4*x^7/(1+x^8 - 4*x^8/(...))))))))), a continued fraction.

G.f.: A(x) = (1 + x*B(x))/(1 - 3*x*B(x)), where B(x) = (1 + x^2*C(x))/(1 - 3*x^2*C(x)), C(x) = (1 + x^3*D(x))/(1 - 3*x^3*D(x)), D(x) = (1 + x^4*E(x))/(1 - 3*x^4*E(x)), ...

a(n) ~ c * 4^n, where c = 2^(3/4) / EllipticTheta[2, 0, 1/2] = 0.789970474669932371974378022396788915338046391238667... - Vaclav Kotesovec, Oct 18 2020, updated Mar 17 2024

Example

G.f.: A(x) = 1 + 4*x + 12*x^2 + 52*x^3 + 204*x^4 + 804*x^5 + 3244*x^6 +...
such that
log(A(x)) = 4*x/(1+x) + 4^2*x^2/(2*(1+x^2)) + 4^3*x^3/(3*(1+x^3)) + 4^4*x^4/(4*(1+x^4)) + 4^5*x^5/(5*(1+x^5)) +...

Mathematica

nmax = 30; CoefficientList[Series[Exp[Sum[4^k * x^k / (1 + x^k)/k, {k, 1, nmax}]], {x, 0, nmax}], x] (* Vaclav Kotesovec, Oct 18 2020 *)

Prog

(PARI) {a(n)=if(n==0, 1, polcoeff(exp(sum(m=1, n, 4^m*x^m/(1+x^m+x*O(x^n))/m)), n))}
for(n=0, 30, print1(a(n), ", "))
(PARI) {a(n)=local(A=1+x); for(i=1, n, A=(1 + x^(n+1-i)*A)/(1 - 3*x^(n+1-i)*A+ x*O(x^n))); polcoeff(A, n)}
for(n=0, 30, print1(a(n), ", "))

Crossrefs

Cf. A165941, A259273, A259275, A259276.

Sequence in context: A149410 A149411 A149412 * A109499 A282587 A188230

Adjacent sequences: A259271 A259272 A259273 * A259275 A259276 A259277

Keyword

nonn

Author

Paul D. Hanna, Jun 23 2015

Status

approved