Number of permutations p of [n] such that max(|p(p(i)) - i|)=1.

Terms

a(0) =0a(1) =0a(2) =2a(3) =6a(4) =24a(5) =80a(6) =312a(7) =1152a(8) =4616a(9) =18384a(10) =77280a(11) =328352a(12) =1451712a(13) =6527616a(14) =30299504a(15) =143322000a(16) =696359232a(17) =3448656512a(18) =17483842176a(19) =90301959744a(20) =476270369600