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In ternary representation of n, replace 2's with 0's.
9

%I #21 Nov 11 2019 10:27:57

%S 0,1,0,3,4,3,0,1,0,9,10,9,12,13,12,9,10,9,0,1,0,3,4,3,0,1,0,27,28,27,

%T 30,31,30,27,28,27,36,37,36,39,40,39,36,37,36,27,28,27,30,31,30,27,28,

%U 27,0,1,0,3,4,3,0,1,0,9,10,9,12,13,12,9,10,9

%N In ternary representation of n, replace 2's with 0's.

%H Alois P. Heinz, <a href="/A244042/b244042.txt">Table of n, a(n) for n = 0..6560</a>

%F a(n) = n - 2 * A005836(A289814(n) + 1) = A005836(A289813(n) + 1). - _Andrey Zabolotskiy_, Nov 11 2019

%e 16 = 121_3, replacing 2 with 0 gives 101_3 = 10, so a(16) = 10.

%p a:= proc(n) local t, r, i; t, r:= n, 0;

%p for i from 0 while t>0 do

%p r:= r+3^i*(d-> `if`(d=2, 0, d))(irem(t, 3, 't'))

%p od; r

%p end:

%p seq(a(n), n=0..80); # _Alois P. Heinz_, Jun 17 2014

%t Array[FromDigits[IntegerDigits[#, 3] /. 2 -> 0, 3] &, 72, 0] (* _Michael De Vlieger_, Mar 17 2018 *)

%o (Python)

%o from sympy.ntheory.factor_ import digits

%o def a(n):return int("".join(map(str, digits(n, 3)[1:])).replace('2', '0'), 3) # _Indranil Ghosh_, Jun 10 2017

%o (PARI) a(n) = my(d=digits(n, 3)); fromdigits(apply(x->(if (x==2, 0, x)), d), 3); \\ _Michel Marcus_, Jun 10 2017

%Y Cf. A004488, A117966, A005836, A289813, A289814.

%K nonn,base,look

%O 0,4

%A _Joonas Pohjonen_, Jun 17 2014