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 A209880 #13 Feb 16 2025 08:33:16
%S A209880 29,112,233,556,1112,2233,5555,1111,2222,4444,8888,16777,34589,112333,
%T A209880 444455,889999,1788899,1177777,4558889,13444447,77888888,156667777,
%U A209880 233444489,1112278888,11999,11119,1223,4444,8888,16777,34589,112333,444455,889999,1788899
%N A209880 RATS: Reverse Add Then Sort the digits applied to previous term, starting with 29.
%C A209880 A114616(1) = 29 is the smallest starting number for a RATS trajectory leading to a cycle of length 18: A114611(29) = 18;
%C A209880 a(n + 18) = a(n) for n > 9.
%H A209880 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RATSSequence.html">RATS Sequence</a>
%H A209880 <a href="/index/Rec#order_18">Index entries for linear recurrences with constant coefficients</a>, signature (0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1).
%F A209880 a(n + 1) = A036839(a(n)).
%t A209880 NestList[FromDigits[Sort[IntegerDigits[#+IntegerReverse[#]]]]&,29,40] (* or *) PadRight[{29,112,233,556,1112,2233,5555,1111,2222},50,{4558889,13444447,77888888,156667777,233444489,1112278888,11999,11119,1223,4444,8888,16777,34589,112333,444455,889999,1788899,1177777}] (* _Harvey P. Dale_, Sep 17 2018 *)
%o A209880 (Haskell)
%o A209880 a209880 n = a209880_list !! (n-1)
%o A209880 a209880_list = iterate a036839 29
%Y A209880 Cf. A004000, A066710, A209878, A066711, A209879.
%K A209880 nonn,base
%O A209880 1,1
%A A209880 _Reinhard Zumkeller_, Mar 14 2012