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 A126304 #8 Jul 24 2025 19:08:32 %S A126304 0,0,0,1,0,1,1,2,1,0,1,1,2,1,1,2,2,3,2,1,2,1,2,0,1,1,2,1,1,2,2,3,2,1, %T A126304 2,1,2,1,2,2,3,2,2,3,3,4,3,2,3,2,3,1,2,2,3,2,1,2,1,2,2,3,2,3,2,0,1,1, %U A126304 2,1,1,2,2,3,2,1,2,1,2,1,2,2,3,2,2,3,3,4,3,2,3,2,3,1,2,2,3,2,1,2,1,2 %N A126304 a(n) = number of nodes with nonzero even distance to the root in the n-th plane general tree encoded by A014486(n). %e A126304 A014486(27) = 696 (1010111000 in binary), encodes the following general plane tree, where the root is marked with * and nodes with even or odd distance to root with 'e's and 'o's, respectively. %e A126304 .......o %e A126304 .......| %e A126304 .......e %e A126304 .......| %e A126304 ...o.o.o %e A126304 ....\|/. %e A126304 .....*.. %e A126304 there is one node marked with 'e', thus a(27)=1. %o A126304 (Scheme) (define (A126304 n) (*A126304 (A014486->parenthesization (A014486 n)))) %o A126304 (define (*A126304 s) (cond ((null? s) 0) (else (apply + (map *A126303 s))))) %Y A126304 a(n) = A126305(n)-1. Cf. A126303. Scheme-function A014486->parenthesization given in A014486. %K A126304 nonn %O A126304 0,8 %A A126304 _Antti Karttunen_, Jan 02 2007