OFFSET
0,6
LINKS
Piera Manara and Claudio Perelli Cippo, The fine structure of 4321 avoiding involutions and 321 avoiding involutions, PU. M. A. Vol. 22 (2011), 227-238; See Theorem 4.2.
FORMULA
Manara and Cippo give a g.f.
MAPLE
A217525 := proc(nmax)
local x ;
-4+4/(1+x^2)+1/(1+x)^2 ;
1/(1+x)-2*x^2*(1+x)-sqrt(%) ;
taylor(%/2, x=0, nmax+1) ;
seq(coeftayl(%, x=0, n), n=0..nmax) ;
end proc: # R. J. Mathar, Nov 05 2012
MATHEMATICA
a[n_] := SeriesCoefficient[(1/2)(-2(x+1)x^2 - Sqrt[4/(x^2+1) + 1/(x+1)^2 - 4] + 1/(x+1)), {x, 0, n}];
a /@ Range[0, 40] (* Jean-François Alcover, Apr 04 2020 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Oct 13 2012
STATUS
approved