A158260 a(n) = coefficient of x^n in the [2^(n-2)]-th iteration of x+x^2 for n>=2.

1, 2, 30, 2240, 685160, 810903456, 3683654668512, 64657586863790400, 4426882384548801561472, 1192052411353154132337483776, 1270174918862853255008627489608704

Offset: 2

Author

Paul D. Hanna, Mar 15 2009

Keywords

• nonn

Examples

			Table of coefficients in the (2^i)-th iteration of x+x^2 begins:
1,(1);
1,2,(2),1;
1,4,12,(30),64,118,188,258,302,298,244,162,84,32,8,1;
1,8,56,364,(2240),13188,74760,409836,2179556,11271436,56788112,...;
1,16,240,3480,49280,(685160),9383248,126855288,1695695976,...;
1,32,992,30256,912640,27297360,(810903456),23950328688,...;
1,64,4032,252000,15665664,969917088,59855127360,(3683654668512),...;
1,128,16256,2056384,259445760,32668147008,4106848523904,515600292989376,(64657586863790400),...;
...
where the terms in parenthesis form the initial terms of this sequence.
		

Crossrefs

Cf. A158261, A158262, A158263, A158264 (table).

Programs

PARI

{a(n)=local(G=x+x^2+x*O(x^n)); if(n<2, 0, for(i=1, n-2, G=subst(G, x, G)); polcoeff(G, n, x))}