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 A086945 #30 May 01 2025 08:29:41
%S A086945 61,691,6991,69991,699991,6999991,69999991,699999991,6999999991,
%T A086945 69999999991,699999999991,6999999999991,69999999999991,
%U A086945 699999999999991,6999999999999991,69999999999999991,699999999999999991,6999999999999999991,69999999999999999991
%N A086945 a(n) = 7*10^n - 9.
%C A086945 a(n) = k where R(k+9) = 7.
%H A086945 Vincenzo Librandi, <a href="/A086945/b086945.txt">Table of n, a(n) for n = 1..300</a>
%H A086945 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A086945 R(a(n)) = A086940(n).
%F A086945 From _Elmo R. Oliveira_, Apr 30 2025: (Start)
%F A086945 G.f.: x*(20*x+61)/((x-1)*(10*x-1)).
%F A086945 E.g.f.: 2 - 9*exp(x) + 7*exp(10*x).
%F A086945 a(n) = 11*a(n-1) - 10*a(n-2) for n > 2. (End)
%t A086945 Table[10 FromDigits[PadRight[{6},n,9]]+1,{n,20}] (* _Harvey P. Dale_, Aug 14 2021 *)
%o A086945 (Magma) [7*10^n-9: n in [1..25] ]; // _Vincenzo Librandi_, Aug 22 2011
%Y A086945 Cf. A004086, A086940.
%K A086945 nonn,base,easy
%O A086945 1,1
%A A086945 _Ray Chandler_, Jul 24 2003