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 A377207 #20 Nov 20 2024 09:46:52 %S A377207 1,18,99,342,2691,13788,65709,407772,2115981,11108358,63181719, %T A377207 334551402,1802963871,9931645728,53256984129,288681869232, %U A377207 1572458030361,8484410567898,46019764248939,249748559819262,1351163694059451,7326501636596868,39716608228492149,215099382176679492 %N A377207 Number of n-digit numbers where every digit is either a 9 or adjacent to a 9. %C A377207 The 9 in the definition can also be any other nonzero digit. %H A377207 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,9,81). %F A377207 G.f.: x*(1 + 8*x)*(1 + 9*x)/(1 - x - 9*x^2 - 81*x^3). - _Andrew Howroyd_, Oct 20 2024 %e A377207 The a(1) = 1 number is 9. %e A377207 The a(2) = 18 numbers are 19, 29, 39, 49, 59, 69, 79, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99. %o A377207 (PARI) Vec((1 + 8*x)*(1 + 9*x)/(1 - x - 9*x^2 - 81*x^3) + O(x^25)) \\ _Andrew Howroyd_, Oct 20 2024 %K A377207 nonn,base,easy %O A377207 1,2 %A A377207 _Edwin McCravy_, Oct 19 2024 %E A377207 a(9) onwards from _Andrew Howroyd_, Oct 20 2024