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 A083813 #12 Jan 25 2020 17:23:01
%S A083813 27,297,2997,29997,299997,2999997,29999997,299999997,2999999997,
%T A083813 29999999997,299999999997,2999999999997,29999999999997,299999999999997
%N A083813 a(n) = 3*(10^n-1).
%C A083813 Original definition: 3n+1 is the digit reversal of n+1.
%C A083813 1. a(n) = 27 + 270 + 2700 + ... up to n terms = sum of n terms of the geometric progression with the first term 27 and common ratio 10.
%C A083813 2. a(n) = 27*A000042(n) (the unary sequence).
%C A083813 Equals A086574 restricted to positive indices. See that entry for many more comments, formulas and references. - _M. F. Hasler_, Jul 29 2016
%t A083813 3(10^Range[20]-1) (* or *) Table[10 FromDigits[PadRight[{2},n,9]]+7,{n,20}] (* _Harvey P. Dale_, Jan 25 2020 *)
%Y A083813 Essentially a duplicate of A086574.
%Y A083813 Cf. A000042, A083811, A083812.
%K A083813 nonn
%O A083813 1,1
%A A083813 _Amarnath Murthy_ and Meenakshi Srikanth (menakan_s(AT)yahoo.com), May 08 2003
%E A083813 Edited by _M. F. Hasler_, Jul 29 2016