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 A118311 #14 Mar 20 2020 04:24:14 %S A118311 1,1,2,4,11,29,77,202,532,1395,3664,9605,25192,66047,173183,453998, %T A118311 1190259,3120294,8180124,21444290,56217025,147373441,386342414, %U A118311 1012799936,2655067412,6960281083,18246444362,47833200849,125395149294,328724391241,861753701567,2259094233704 %N A118311 Number of dissimilar squarefree quaternary words of length n. %C A118311 Sherman Stein and A006156 count ordered squarefree(twin-free) ternary words. A060688 counts the dissimilar cases essentially by dividing by 3! (the number of ways to permute a,b,c). A051041 counts ordered squarefree quaternary words. A118311 counts the dissimilar cases (beginning with the 4th term) by dividing A051041 by 4!. %D A118311 Sherman Stein, How The Other Half Thinks, 2001, page 149. %e A118311 a(1) = 1 because a,b,c and d are similar. %e A118311 a(2) = 1 because aa is not squarefree; so ab is the only valid case. %e A118311 a(3) = 2 counting aba and abc. %e A118311 a(4) = 4 counting abac, abca, abcb and abcd. %e A118311 a(5) = 11 counting abaca,abacb,abcab,abcac,abcba,abacd,abcad,abcbd,abcda,abcdb and abcdc. %Y A118311 Cf. A006156, A060688, A051041. %K A118311 nonn %O A118311 1,3 %A A118311 _Alford Arnold_, Apr 22 2006 %E A118311 a(16)-a(25) from _Max Alekseyev_, Jul 03 2006 %E A118311 a(26)-a(30) from _Giovanni Resta_, Mar 20 2020