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 A199683 #23 Jun 10 2025 16:33:11
%S A199683 4,31,301,3001,30001,300001,3000001,30000001,300000001,3000000001,
%T A199683 30000000001,300000000001,3000000000001,30000000000001,
%U A199683 300000000000001,3000000000000001,30000000000000001,300000000000000001,3000000000000000001,30000000000000000001,300000000000000000001
%N A199683 a(n) = 3*10^n + 1.
%H A199683 Vincenzo Librandi, <a href="/A199683/b199683.txt">Table of n, a(n) for n = 0..1000</a>
%H A199683 S. W. Golomb, <a href="http://www.jstor.org/stable/2005337">Properties of the sequence 3.2^n+1</a>, Math. Comp., 30 (1976), 657-663.
%H A199683 S. W. Golomb, <a href="/A004119/a004119.pdf">Properties of the sequence 3.2^n+1</a>, Math. Comp., 30 (1976), 657-663. [Annotated scanned copy]
%H A199683 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A199683 a(n) = 10*a(n-1) - 9.
%F A199683 a(n) = 11*a(n-1) - 10*a(n-2).
%F A199683 G.f.: (4-13*x)/((1-x)*(1-10*x)).
%F A199683 E.g.f.: exp(x)*(1 + 3*exp(9*x)). - _Elmo R. Oliveira_, Jun 10 2025
%t A199683 3*10^Range[0,20]+1 (* or *) LinearRecurrence[{11,-10},{4,31},20] (* _Harvey P. Dale_, Dec 12 2016 *)
%o A199683 (Magma) [3*10^n+1: n in [0..30]];
%Y A199683 Primes: A056807, A259866.
%K A199683 nonn,easy
%O A199683 0,1
%A A199683 _Vincenzo Librandi_, Nov 09 2011