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 A271220 #22 Apr 12 2025 18:12:17
%S A271220 6,66,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,
%T A271220 1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,
%U A271220 1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236,1236
%N A271220 Concatenate sum of digits of previous term and product of digits of previous term, starting with 6.
%C A271220 Each term is created by calculating the sum of the digits of the previous number, and the product of its digits. The results are concatenated to give the new number. Starting with 6, the second number is 66. The third number is generated as follows: 6+6 = 12, 6*6 = 36, which gives 1236. After that, the numbers remain unchanged, because 1+2+3+6 = 12 and 1x2x3x6 = 36, so combined 1236 again.
%C A271220 For more information, see A380873 (the iterated function), A380872 (all trajectories), A062237 (fixed points). - _M. F. Hasler_, Apr 02 2025
%H A271220 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (1).
%F A271220 a(n) = 1236 for all n > 2. - _M. F. Hasler_, Apr 02 2025
%t A271220 NestList[FromDigits[Flatten@ {IntegerDigits@ Total@ #, IntegerDigits@ If[Length@ # == 1, #, Times @@ #]}] &@ IntegerDigits@ # &, 6, 50] (* _Michael De Vlieger_, Apr 02 2016 *)
%o A271220 (PARI) A380872_row(6) \\ _M. F. Hasler_, Apr 02 2025
%Y A271220 Cf. A380873 (the iterated function), A007953 (sum of digits), A007954 (product of digits), A380872 (all trajectories), A062237 (fixed points).
%K A271220 nonn,easy,base
%O A271220 0,1
%A A271220 _Sander Claassen_, Apr 02 2016
%E A271220 Offset changed to 0 by _M. F. Hasler_, Apr 02 2025