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 A105186 #12 May 20 2020 05:04:59 %S A105186 0,1,2,0,1,2,0,1,2,9,10,11,9,10,11,9,10,11,18,19,20,18,19,20,18,19,20, %T A105186 0,1,2,0,1,2,0,1,2,9,10,11,9,10,11,9,10,11,18,19,20,18,19,20,18,19,20, %U A105186 0,1,2,0,1,2,0,1,2,9,10,11,9,10,11,9,10,11,18,19,20,18,19,20,18,19,20,81,82 %N A105186 Replace odd-positioned digits with 0 in ternary representation of n. %H A105186 Reinhard Zumkeller, <a href="/A105186/b105186.txt">Table of n, a(n) for n = 0..10000</a> %F A105186 a(n) = n - a(floor(n/3))*3, a(0) = 0. %F A105186 a(n) = 9*a(floor(n/9)) + (n mod 9) mod 3. - _Reinhard Zumkeller_, Sep 26 2015 %e A105186 n = 123 = '11120' --> '10100' = 90 = a(123). %o A105186 (Haskell) %o A105186 a105186 0 = 0 %o A105186 a105186 n = 9 * a105186 n' + mod t 3 %o A105186 where (n', t) = divMod n 9 %o A105186 -- _Reinhard Zumkeller_, Sep 26 2015 %o A105186 (PARI) a(n) = fromdigits(digits(n,9)%3,9); \\ _Kevin Ryde_, May 20 2020 %Y A105186 Cf. A007089, A063694, A163325. %K A105186 nonn,base %O A105186 0,3 %A A105186 _Reinhard Zumkeller_, Apr 11 2005