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 A209641 #14 Jul 01 2025 14:59:38 %S A209641 0,2,10,12,42,50,52,56,170,202,210,212,226,228,232,240,682,810,842, %T A209641 850,852,906,914,916,930,932,936,962,964,968,976,992,2730,3242,3370, %U A209641 3402,3410,3412,3626,3658,3666,3668,3722,3730,3732,3746,3748,3752,3850,3858,3860,3874,3876,3880,3906,3908,3912,3920,3970,3972 %N A209641 A014486-codes for rooted plane trees where non-leaf branches can occur only as the leftmost branch of any level, but nowhere else. Sorted into ascending order. %H A209641 Antti Karttunen, <a href="/A209641/b209641.txt">Table of n, a(n) for n = 0..32767</a> %o A209641 (Scheme) (define (member_of_A209641? n) (let loop ((n n) (lev 0)) (cond ((zero? n) (zero? lev)) ((< lev 0) #f) ((even? n) (loop (/ n 2) (+ lev 1))) ((and (odd? (/ (-1+ n) 2)) (even? (/ (-1+ (/ (-1+ n) 2)) 2)) (not (zero? (/ (-1+ (/ (-1+ n) 2)) 2)))) #f) (else (loop (/ (- n 1) 2) (- lev 1)))))) %o A209641 (define A209641 (MATCHING-POS 0 0 member_of_A209641?)) ;; MATCHING-POS in AK's Scheme-SeqFun-Transform package. %Y A209641 A209640(a(n)) = n for all n. a(n) = A014486(A209643(n)). Sequence A209642 sorted into ascending order with permutation A209862, i.e. a(n) = A209642(A209862(n)). %K A209641 nonn %O A209641 0,2 %A A209641 _Antti Karttunen_, Mar 24 2012