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 A177019 #19 Aug 20 2024 15:26:21
%S A177019 7,331,30301,3003001,300030001,30000300001,3000003000001,
%T A177019 300000030000001,30000000300000001,3000000003000000001,
%U A177019 300000000030000000001,30000000000300000000001,3000000000003000000000001,300000000000030000000000001,30000000000000300000000000001,3000000000000003000000000000001
%N A177019 a(n) = 3*10^(2*n) + 3*10^n + 1.
%H A177019 Vincenzo Librandi, <a href="/A177019/b177019.txt">Table of n, a(n) for n = 0..100</a>
%H A177019 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A177019 G.f.: (7-446*x+1330*x^2)/((1-x)*(1-10*x)*(1-100*x)). - _Vincenzo Librandi_, Aug 19 2014
%F A177019 E.g.f.: exp(x)*(1 + 3*exp(9*x) + 3*exp(99*x)). - _Stefano Spezia_, Aug 05 2024
%e A177019 For n=0, a(0)=7; n=1, a(1)=3*10^2+3*10+1=331.
%t A177019 CoefficientList[Series[(7 - 446 x + 1330 x^2)/((1 - x)(1 - 10 x) (1 - 100 x)), {x, 0, 20}], x] (* _Vincenzo Librandi_, Aug 19 2014 *)
%o A177019 (Magma) [(3*10^(2*n)+3*10^n+1): n in [0..15]];
%Y A177019 Cf. A011557, A098608.
%K A177019 nonn,easy
%O A177019 0,1
%A A177019 _Vincenzo Librandi_, May 24 2010