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 A199495 #23 Nov 05 2024 08:36:37 %S A199495 0,1,0,2,4,36,144,1440,8640,100800,806400,10886400,108864000, %T A199495 1676505600,20118067200,348713164800,4881984307200,94152554496000, %U A199495 1506440871936000,32011868528640000,576213633515520000,13380961044971520000,267619220899430400000 %N A199495 Number of permutations of [n] starting and ending with an odd number. %H A199495 Alois P. Heinz, <a href="/A199495/b199495.txt">Table of n, a(n) for n = 0..450</a> %F A199495 From _Alois P. Heinz_, Nov 07 2011: (Start) %F A199495 a(n) = 2*(n-2)! * [x^(n-2)] x/((1+x)^2*(1-x)^3) for n>=2, else a(n) = n. %F A199495 a(n) = 2*A152887(n-2) for n>=2, else a(n) = n. (End) %p A199495 a:= n-> `if`(n<2, n, ceil(n/2)*(ceil(n/2)-1)*(n-2)!): %p A199495 seq(a(n), n=0..30); # _Alois P. Heinz_, Nov 07 2011 %Y A199495 Cf. A052618, A077611, A152887. %K A199495 nonn %O A199495 0,4 %A A199495 _Olivier Gérard_, Nov 07 2011