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 A269121 #7 Feb 23 2016 12:23:51 %S A269121 1,5,43,374,3199,26945,224296,1850672,15168751,123700631,1004849563, %T A269121 8137906034,65749071892,530209766156,4269268590280,34334967161024, %U A269121 275866609658095,2214734413345667,17769222356447761,142492124882039978,1142168892887289319 %N A269121 Number of sequences with n copies each of 1,2,3 avoiding the pattern 123. %H A269121 Alois P. Heinz, <a href="/A269121/b269121.txt">Table of n, a(n) for n = 0..1000</a> %F A269121 a(n) = A006480(n) - A047911(n). %e A269121 a(0) = 1: the empty sequence. %e A269121 a(1) = 5: 132, 213, 231, 312, 321. %e A269121 a(2) = 43: 113322, 131322, 133122, 133212, 133221, 211332, 213132, 213312, 213321, 221133, 221313, 221331, 223113, 223131, 223311, 231132, 231312, 231321, 232113, 232131, 232311, 233112, 233121, 233211, 311322, 313122, 313212, 313221, 321132, 321312, 321321, 322113, 322131, 322311, 323112, 323121, 323211, 331122, 331212, 331221, 332112, 332121, 332211. %Y A269121 Row n=3 of A269129. %Y A269121 Cf. A006480, A047911. %K A269121 nonn %O A269121 0,2 %A A269121 _Alois P. Heinz_, Feb 19 2016