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 A209879 #16 Feb 16 2025 08:33:16
%S A209879 6999,15699,11355,66666,123333,445566,111111,222222,444444,888888,
%T A209879 1677777,3455589,11112333,33444444,77778888,156666666,123333378,
%U A209879 666669999,1356666666,123333789,11111667,22777788,11115555,66666666,123333333,445566666,111122223
%N A209879 RATS: Reverse Add Then Sort the digits applied to previous term, starting with 6999.
%C A209879 A114615(1) = 6999 is the smallest starting number for a RATS trajectory leading to a cycle of length 14: A114611(6999) = 14;
%C A209879 a(n + 14) = a(n) for n > 25.
%H A209879 Reinhard Zumkeller, <a href="/A209879/b209879.txt">Table of n, a(n) for n = 1..100</a>
%H A209879 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RATSSequence.html">RATS Sequence</a>
%H A209879 <a href="/index/Rec#order_14">Index entries for linear recurrences with constant coefficients</a>, signature (0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1).
%F A209879 a(n + 1) = A036839(a(n)).
%t A209879 rats[n_]:=Module[{idnr=FromDigits[Reverse[IntegerDigits[n]]]}, FromDigits[ Sort[ IntegerDigits[idnr+n]]]]; NestList[rats,6999,30] (* _Harvey P. Dale_, May 29 2014 *)
%o A209879 (Haskell)
%o A209879 a209879 n = a209879_list !! (n-1)
%o A209879 a209879_list = iterate a036839 6999
%Y A209879 Cf. A004000, A066710, A209878, A066711, A209880.
%K A209879 nonn,base
%O A209879 1,1
%A A209879 _Reinhard Zumkeller_, Mar 14 2012