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 A341704 #9 Mar 07 2021 19:49:10 %S A341704 1,1,1,3,9,21,61,211,673,1849,4441,9571,18921,34893,60789,101011, %T A341704 161281,248881,372913,544579,777481,1087941,1495341,2022483,2695969, %U A341704 3546601,4609801,5926051,7541353,9507709,11883621,14734611,18133761,22162273,26910049,32476291 %N A341704 a(n) = 20*binomial(n,6) + 2*binomial(n,3) + 1. %C A341704 a(n) is the number of ternary strings of length n that contain either none or three 0's and either none or three 1's. %H A341704 <a href="/index/Rec#order_07">Index entries for linear recurrences with constant coefficients</a>, signature (7,-21,35,-35,21,-7,1). %F A341704 E.g.f.: exp(x)*(1 + x^3/6)^2. %F A341704 O.g.f.:(1 - 6*x + 15*x^2 - 18*x^3 + 9*x^4 + 19*x^6)/(1 - x)^7. - _Stefano Spezia_, Feb 19 2021 %e A341704 a(7)=211 since the strings are the 140 permutations of 0001112, the 35 permutations of 0002222, the 35 permutations of 1112222, and 2222222. %Y A341704 Cf. A341703, A341705. %K A341704 nonn,easy %O A341704 0,4 %A A341704 _Enrique Navarrete_, Feb 17 2021