OFFSET
0,2
FORMULA
E.g.f. satisfies: A(x) = (1-x+x^2)^2/(1-2x)*A(x-x^2+x^3).
EXAMPLE
E.g.f.: A(x) = 1 + 3x + 16x^2/2! + 120x^3/3! + 1140x^4/4! + 12972x^5/5! +...
PROG
(PARI) {a(n)=if(n<0, 0, if(n==0, 1, (n-1)!*sum(i=1, n, (-1)^(i+1)*polcoeff((1+x+x^2)^(n-i+2), i+1)*a(n-i)/(n-i)!)))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Dec 27 2007, Jan 24 2008
STATUS
approved