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 A199682 #27 Nov 26 2023 04:13:42
%S A199682 3,21,201,2001,20001,200001,2000001,20000001,200000001,2000000001,
%T A199682 20000000001,200000000001,2000000000001,20000000000001,
%U A199682 200000000000001,2000000000000001,20000000000000001,200000000000000001,2000000000000000001
%N A199682 a(n) = 2*10^n + 1.
%C A199682 Numbers k such that (R(k) - 1)/(k + 1) = 1/2, where R(k) denotes the digit reversal of k (cf. A004086). - _Stefano Spezia_, Nov 25 2023
%H A199682 Vincenzo Librandi, <a href="/A199682/b199682.txt">Table of n, a(n) for n = 0..1000</a>
%H A199682 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A199682 a(n) = 10*a(n-1)-9.
%F A199682 a(n) = 11*a(n-1)-10*a(n-2).
%F A199682 G.f.: 3*(1-4*x)/((1-x)*(1-10*x)).
%F A199682 E.g.f.: 2*exp(10*x) + exp(x). - _Stefano Spezia_, Nov 25 2023
%t A199682 NestList[10#-9&,3,20] (* or *) LinearRecurrence[{11,-10},{3,21},20] (* _Harvey P. Dale_, Sep 30 2017 *)
%o A199682 (Magma) [2*10^n+1: n in [0..30]];
%o A199682 (Haskell)
%o A199682 a199682 = (+ 1) . (* 2) . (10 ^) -- _Reinhard Zumkeller_, Jan 30 2015
%o A199682 (PARI) a(n)=2*10^n+1 \\ _Charles R Greathouse IV_, Oct 16 2015
%Y A199682 Cf. A004086, A011557, A133384.
%K A199682 nonn,easy
%O A199682 0,1
%A A199682 _Vincenzo Librandi_, Nov 09 2011