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 A177285 #11 May 30 2025 16:57:36 %S A177285 1,1,6,90,2520,113400,3356640,75336660,1500131820,29660400822, %T A177285 620330067782,13125403608162,274194614811528,5605288189594200, %U A177285 112437639146871360,2235073249396108440,44209749453949113840,870798759948756386976,17069437222534487122608,332789644153015982087550,6455422726721469064982952 %N A177285 Number of permutations of 2 copies of 1..n with all adjacent differences <= 4 in absolute value. %F A177285 a(n) = (2n)!/2^n for n<=5. %Y A177285 Cf. A177283. %K A177285 nonn %O A177285 0,3 %A A177285 _R. H. Hardin_, May 06 2010 %E A177285 a(0)=1 prepended and terms a(13) and beyond from _Andrew Howroyd_, May 14 2020