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 A177287 #8 May 30 2025 16:57:32 %S A177287 1,1,6,90,2520,113400,7484400,681080400,46118872800,2475822056400, %T A177287 113874710498640,4828589411913540 %N A177287 Number of permutations of 2 copies of 1..n with all adjacent differences <= 6 in absolute value. %F A177287 a(n) = (2n)!/2^n for n<=7. %K A177287 nonn,more %O A177287 0,3 %A A177287 _R. H. Hardin_, May 06 2010 %E A177287 a(0)=1 prepended by _Alois P. Heinz_, Jul 21 2018