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 A089848 #6 Mar 31 2012 14:02:17 %S A089848 1,1,2,2,5,15,42,132,431,1430,4862,16801,58786,208012,742914,2674440, %T A089848 9694845,35357712,129644790,477638700,1767263322,6564120420, %U A089848 24466267020,91482564069,343059613650,1289904147324,4861946402882,18367353072152 %N A089848 Number of fixed points in range [A014137(n-1)..A014138(n-1)] of permutation A089851/A089853. %C A089848 The number of n-node binary trees fixed by the corresponding automorphism(s). %H A089848 A. Karttunen, <a href="/A089408/a089408.c.txt">C-program for computing the initial terms of this sequence</a> %F A089848 a(0)=1, a(n) = A000108(n-1) if (n mod 3)=0 or 1 and a(n) = A000108(n-1)+A000108((n-2)/3) if (n mod 3)=2. %p A089848 A089848 := n -> A000108(n-1)+`if`((2 = (n mod 3)), A000108((n-2)/3),0); %o A089848 (Scheme) (define (A089848 n) (cond ((zero? n) 1) (else (+ (A000108 (- n 1)) (if (= 2 (modulo n 3)) (A000108 (/ (- n 2) 3)) 0))))) %K A089848 nonn,easy %O A089848 0,3 %A A089848 _Antti Karttunen_, Nov 29 2003