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%I #18 Sep 17 2019 08:31:05
%S 0,1,2,3,4,7,6,5,8,9,10,19,12,13,22,21,16,25,18,11,20,15,14,23,24,17,
%T 26,27,28,55,30,31,58,57,34,61,36,37,64,39,40,67,66,49,76,63,46,73,48,
%U 43,70,75,52,79,54,29,56,33,32,59,60,35,62,45,38,65,42,41
%N Reverse nonzero digits in ternary expansion of n and convert back to decimal.
%C This sequence is a self-inverse permutation of nonnegative integers with fixed points A321473.
%C See A321474 for the decimal variant.
%C The binary variant simply corresponds to the identity (A001477).
%H Rémy Sigrist, <a href="/A321464/b321464.txt">Table of n, a(n) for n = 0..19683</a>
%H <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a>
%F a(3 * n) = 3 * a(n).
%e The first values at prime indices, alongside the corresponding ternary expansions, are:
%e n a(n) ter(n) ter(a(n))
%e -- ---- ------ ---------
%e 2 2 2 2
%e 3 3 10 10
%e 5 7 12 21
%e 7 5 21 12
%e 11 19 102 201
%e 13 13 111 111
%e 17 25 122 221
%e 19 11 201 102
%e 23 23 212 212
%e 29 55 1002 2001
%e 31 31 1011 1011
%e 37 37 1101 1101
%e 41 67 1112 2111
%e 43 49 1121 1211
%t a[n_] := Block[{x = IntegerDigits[n, 3], t}, t = Flatten@ Position[x, 1 | 2]; x[[Reverse@ t]] = x[[t]]; FromDigits[x, 3]]; Array[a, 68, 0] (* _Giovanni Resta_, Sep 17 2019 *)
%o (PARI) a(n,base=3) = my (d=digits(n,base),t=Vecrev(select(sign,d)),i=0); for (j=1, #d, if (d[j], d[j] = t[i++])); fromdigits(d,base)
%Y Cf. A001477, A004488, A030102, A321473, A321474.
%K nonn,base,look
%O 0,3
%A _Rémy Sigrist_, Nov 10 2018
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