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 A335551 #34 Sep 16 2020 07:44:42
%S A335551 0,0,1,5,18,58,177,522,1503,4252,11869,32787,89821,244415,661415,
%T A335551 1781654,4780776,12786704,34104792,90749209,240982564,638800052,
%U A335551 1690764378,4469170031,11799684559,31122693066,82016622160,215969175981,568313267862,1494601936229
%N A335551 Number of words of length n over the alphabet {0,1,2} that contain the substring 12 but not the substring 01.
%H A335551 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (6,-12,10,-5,1).
%F A335551 a(n) = Sum_{i=1..n} A001906(n-i) * A052921(i-1).
%F A335551 G.f.: x^2*(x-1)/((x^2-3*x+1)*(x^3-2*x^2+3*x-1)). - _Alois P. Heinz_, Sep 15 2020
%e A335551 a(0) = a(1) = 0, because no word of length n < 2 can contain 12.
%e A335551 a(2) = 1, because there is one word of length 2 and it is 12.
%e A335551 a(3) = 5, because there are 5 words of length 3 and they are 121, 112, 212, 122, 120.
%Y A335551 Cf. A001906, A052921.
%K A335551 nonn,easy
%O A335551 0,4
%A A335551 _Mauricio J. Santos_, Sep 15 2020
%E A335551 a(20)-a(29) from _Alois P. Heinz_, Sep 15 2020