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 A081609 #10 Jun 25 2022 10:01:48 %S A081609 0,1,1,2,3,4,4,5,5,6,7,8,9,10,11,12,13,14,14,15,15,16,17,18,18,19,19, %T A081609 20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42, %U A081609 43,44,45,46,46,47,47,48,49,50,50,51,51,52,53,54,55,56,57,58,59,60,60,61 %N A081609 Number of numbers <= n having at least one 1 in their ternary representation. %C A081609 a(n) + A061392(n) = n+1. %o A081609 (PARI) has(n)=!!setsearch(Set(digits(n,3)),1) %o A081609 first(n)=my(s); vector(n,k,s+=has(k)) \\ _Charles R Greathouse IV_, Sep 02 2015 %Y A081609 Cf. A007089, A062756, A081607, A081610, A061392. %Y A081609 Partial sums of A316829. %K A081609 nonn %O A081609 0,4 %A A081609 _Reinhard Zumkeller_, Mar 23 2003