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%I #27 Feb 11 2019 13:27:47
%S 1,3,9,27,81,243,729,2187,6561,51,153,459,1377,13,39,117,7,21,63,189,
%T 567,1701,70,210,630,1890,5670,17010,7,21,63,189,567,1701,70,210,630,
%U 1890,5670,17010,7,21,63,189,567,1701,70,210,630,1890,5670,17010,7,21,63,189,567,1701,70,210,630,1890,5670,17010,7
%N Trajectory of the "Erase or triple" protocol applied to 1 (see Comments section for how the protocol works).
%C The "Erase or triple" protocol describes how to transform an integer K into an integer L: if K has 2 or more identical digits, erase them to get L (1201331 becomes 20); if K has no duplicate digits, triple K to get L (20 becomes 60).
%C Some integers disappear immediately (like 11, 2002 or 1919188 - see the Crossref section), other enter into a loop if you apply this protocol to the successive results.
%C Note that 1102 is transformed into 2 because no leading zeroes are admitted.
%C It seems that all integers up to 10^6 enter into one of those four loops:
%C Loop zero: 0, 0, 0, 0, 0,...
%C Loop five: 5, 15, 45, 135, 405, 1215, 25, 75, 225, 5, ...
%C Loop seven: 7, 21, 63, 189, 567, 1701, 70, 210, 630, 1890, 5670, 17010, 7, ...
%C Loop eighty-nine : 89, 267, 801, 2403, 7209, 21627, 167, 501,1503, 4509, 13527, 40581, 121743, 2743, 8229, 89, ...
%C Since L <= 9876543210 if K > 9876543210, it suffices to only look at trajectories for 0 <= K <= 9876543210. Exhaustive search shows every trajectory converges to one of the four loops above. - _Chai Wah Wu_, Feb 11 2019
%e 1 becomes 3 (the triple of 1), then 3 becomes 9 (the triple of 3), then 6 becomes 27 (the triple of 9) and now 81, 243, 729, 2187, 6561, 51 (because the pair 6-6 is erased), 153 (the triple of 54), ... until 17010 which will turn into a term already in the sequence (7). At this point the trajectory cycles.
%t NestWhileList[Function[n, If[#1 == #2, 3 FromDigits@ #1, FromDigits@ #2] & @@ {#, Select[#, DigitCount[n, 10, #] == 1 &]} &@ IntegerDigits@ n], 1, Unequal, All] (* _Michael De Vlieger_, Feb 28 2018 *)
%Y Cf. A300149 which shows the first 33333 integers that disappear.
%K nonn,base
%O 1,2
%A _Eric Angelini_ and _Jean-Marc Falcoz_, Feb 27 2018