OFFSET
0,1
COMMENTS
Sum_{n>=0} 1/a(n) = Pi/4.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..1000
X. Gourdon and P. Sebah, The constant Pi. The classic period
FORMULA
G.f.: 2*(1-4*x^2)/(1+4*x^2)^2 + 3*x*(1-9*x^2)/(1+9*x^2)^2.
MATHEMATICA
CoefficientList[Series[2 (1 - 4 x^2)/(1 + 4 x^2)^2 + 3 x (1 - 9 x^2)/(1 + 9 x^2)^2, {x, 0, 40}], x] (* Vincenzo Librandi, Dec 12 2012 *)
CROSSREFS
KEYWORD
frac,sign
AUTHOR
Jaume Oliver Lafont, Feb 27 2009
STATUS
approved