This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A122894 #3 Mar 30 2012 18:36:59
%S A122894 1,2,9,258,293685,531124770570,2439717292075827330588969,
%T A122894 72554628124279239546273779187960042205300343234178
%N A122894 Coefficient of x^(2^(n-1)) in the n-th self-composition of (x+x^2) for n>=1.
%C A122894 Originated by Ralf Stephan in A092123 as the 2^(n-1)th coefficient in the expansion of P(0)=x, P(n+1)=P(n)*[1+P(n)] (equivalent definition). Next term is too large to include.
%e A122894 a(1) = 1 = [x^1] (x + x^2).
%e A122894 a(2) = 2 = [x^2] (x + 2*x^2 + 2*x^3 + x^4).
%e A122894 a(3) = 9 = [x^4] (x + 3*x^2 + 6*x^3 + 9*x^4 + 10*x^5 + 8*x^6 + 4*x^7 + x^8).
%o A122894 (PARI) {a(n)=local(F=x+x^2, G=x+x*O(x^(2^(n-1)))); if(n<1, 0, for(i=1, n, G=subst(F, x, G)); return(polcoeff(G, 2^(n-1), x)))}
%Y A122894 Cf. A092123, A122893; A122888.
%K A122894 nonn
%O A122894 1,2
%A A122894 _Paul D. Hanna_, Sep 19 2006