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 A154125 #7 Dec 13 2015 17:38:25 %S A154125 0,1,2,3,4,5,6,7,8,9,10,11,12,13,16,19,14,17,18,15,20,21,22,23,24,25, %T A154125 26,27,28,29,30,31,32,33,34,35,36,44,47,53,56,60,42,51,37,45,46,38,48, %U A154125 49,50,43,52,39,54,55,40,57,58,59,41,61,62,63,64,65,66,67,68,69,70,71 %N A154125 Self-inverse signature permutation of a Catalan bijection: row 83 of A089840. %C A154125 This bijection of binary trees swaps the left and right subtree of a binary tree, but ONLY if BOTH are nonempty. If either the left or right hand side tree is empty, fixes the tree. %C A154125 .A...B.C...D.......C...D.A...B. %C A154125 ..\./...\./.........\./...\./.. %C A154125 ...x.....x...--->....x.....x... %C A154125 ....\.../.............\.../.... %C A154125 ......x.................x...... %C A154125 ............................... %C A154125 ((a . b) . (c . d)) -> ((c . d) . (a . b)) %C A154125 or fix, if either the left or right hand side subtree is empty. %H A154125 A. Karttunen, <a href="/A154125/b154125.txt">Table of n, a(n) for n = 0..2055</a> %H A154125 <a href="/index/Per#IntegerPermutationCatAuto">Index entries for signature-permutations of Catalan automorphisms</a> %o A154125 (Destructive version of this automorphism in Scheme:) (define (*A154125! s) (if (and (pair? s) (pair? (car s)) (pair? (cdr s))) (*A069770! s)) s) %Y A154125 Inverse: A154125. a(n) = A069770(A154126(n)) = A154126(A069770(n)). %K A154125 nonn %O A154125 0,3 %A A154125 _Antti Karttunen_, Jan 06 2009