A105186 - OEIS

%I #12 May 20 2020 05:04:59

%S 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 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 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 Replace odd-positioned digits with 0 in ternary representation of n.

%H Reinhard Zumkeller, <a href="/A105186/b105186.txt">Table of n, a(n) for n = 0..10000</a>

%F a(n) = n - a(floor(n/3))*3, a(0) = 0.

%F a(n) = 9*a(floor(n/9)) + (n mod 9) mod 3. - _Reinhard Zumkeller_, Sep 26 2015

%e n = 123 = '11120' --> '10100' = 90 = a(123).

%o (Haskell)

%o a105186 0 = 0

%o a105186 n = 9 * a105186 n' + mod t 3

%o             where (n', t) = divMod n 9

%o -- _Reinhard Zumkeller_, Sep 26 2015

%o (PARI) a(n) = fromdigits(digits(n,9)%3,9); \\ _Kevin Ryde_, May 20 2020

%Y Cf. A007089, A063694, A163325.

%K nonn,base

%O 0,3

%A _Reinhard Zumkeller_, Apr 11 2005