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 A187131 #7 Jul 22 2025 10:39:19 %S A187131 1,1,9,1521,71622369,233297499911422401, %T A187131 3390052406222940758260506721830900609, %U A187131 934785860242188709610961043825803400592180434378516146129897302939414193921 %N A187131 Numerator of probability that the height of a rooted random binary tree is n. %C A187131 If each node of a rooted random binary tree has probability 1/2 of producing two branches, and p(n) is the probability that the height of the tree is n, then p(n) has the following properties: %C A187131 * p(n) = 2*b(n+1)^2 with b(n) defined as in A076628; %C A187131 * p(n+1) = p(n)*(1 - sqrt(p(n)/2))^2 starting from p(0)=1/2; %C A187131 * Sum_n p(n) = 1; %C A187131 * Sum_n n*p(n) is infinite; %C A187131 * p(n) = a(n) / 2^(2^(n+1)-1). %F A187131 a(n) = A076628(n)^2. %e A187131 For n=0 the root node may have no branches giving the tree height 0, so p(0)=1/2 and a(0)=1; p(1) = 1/2*1/4 = 1/8 so a(1)=1; p(2) = 1/4*1/4 + 1/8*1/16 = 9/128 so a(2)=9; p(3) = 5/32*1/4 + 7/64*1/16 + 1/32*1/64 + 1/128*1/256 = 1521/32768 so a(3)=1521. %Y A187131 Denominator is A058891 offset %K A187131 nonn %O A187131 0,3 %A A187131 _Henry Bottomley_, Mar 05 2011