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 A177291 #17 May 16 2020 01:33:21 %S A177291 1,1,20,92,506,2288,10010,41618,168284,664958,2584442,9916688, %T A177291 37679618,142079906,532572428,1987037318,7386724082,27381500624, %U A177291 101272019258,373902595130,1378571667644,5077289249390,18683930010890,68709775705328,252549056389394,927895845621746 %N A177291 Number of permutations of 3 copies of 1..n with all adjacent differences <= 1 in absolute value. %C A177291 a(n) = (3n)!/6^n = A014606(n) for n<=2. %H A177291 Andrew Howroyd, <a href="/A177291/b177291.txt">Table of n, a(n) for n = 0..200</a> %Y A177291 Column k=3 of A331562. %Y A177291 Cf. A014606. %K A177291 nonn %O A177291 0,3 %A A177291 _R. H. Hardin_, May 06 2010 %E A177291 a(0)=1 prepended by _Alois P. Heinz_, Jan 21 2020 %E A177291 a(19)-a(20) from _Alois P. Heinz_, Jan 22 2020 %E A177291 Terms a(21) and beyond from _Andrew Howroyd_, May 14 2020