OFFSET
0,3
COMMENTS
Equals column 0 and main diagonal of triangle A120894 (cascadence of 1+x+x^2).
EXAMPLE
A(x) = 1 + x + 2*x^2 + 5*x^3 + 12*x^4 + 30*x^5 + 78*x^6 + 206*x^7+...
= G(x)*A(x^3*G(x)^2) where
G(x) = 1 + x + 2*x^2 + 4*x^3 + 9*x^4 + 21*x^5 + 51*x^6 + 127*x^7 +...
is the g.f. of the Motzkin numbers (A001006) so that G(x) satisfies:
G(x) = 1 + x*G(x) + x^2*G(x)^2.
PROG
(PARI) {a(n)=local(A=1+x, G=1/x*serreverse(x/(1+x+x^2+x*O(x^n)))); for(i=0, n, A=G*subst(A, x, x^3*G^2 +x*O(x^n))); polcoeff(A, n, x)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Jul 14 2006
STATUS
approved