OFFSET
0,3
COMMENTS
In general, e.g.f. exp(x(1+ax)/(1-bx)) has general term sum{i=0..n, sum{j=0..n, a^j*b^(n-i-j)*C(i,j)C(n-j-1,n-i-j)*n!/i!}}.
Let k be a positive integer. The sequence obtained by reducing a(n) modulo k is a purely periodic sequence whose period divides k. For example, taken modulo 14 the sequence becomes [1, 1, 7, 13, 3, 7, 5, 1, 1, 7, 13, 3, 7, 5, ...], a periodic sequence of period 7. Cf. A047974. - Peter Bala, Jul 15 2022
FORMULA
E.g.f.: exp(x*(1+x)/(1-2*x)).
a(n) = Sum_{i = 0..n} Sum_{j = 0..n} 2^(n-i-j)*C(i, j)*C(n-j-1, n-i-j)*n!/i!.
a(n) ~ 3^(1/4) * 2^(n-1) * n^(n-1/4) * exp(sqrt(3*n)-n-5/8). - Vaclav Kotesovec, Sep 25 2013
Conjecture: a(n) +(-4*n+3)*a(n-1) +2*(n-1)*(2*n-5)*a(n-2) +2*(n-1)*(n-2)*a(n-3)=0. - R. J. Mathar, Feb 20 2015
The e.g.f. A(x) satisfies the differential equation (4*x^2 - 4*x + 1)*A'(x) + (2*x^2 - 2*x - 1)*A(x) = 0 with A(0) = 1. Mathar's conjectured recurrence follows easily from this. - Peter Bala, Jul 15 2022
MATHEMATICA
With[{nn=20}, CoefficientList[Series[Exp[(x(x+1))/(1-2x)], {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Sep 21 2011 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Paul Barry, Aug 29 2005
STATUS
approved