Number of permutations (p1,...,pn) such that 1 <= |pk - k| <= 2 for all k.

Terms

a(0) =1a(1) =0a(2) =1a(3) =2a(4) =4a(5) =6a(6) =13a(7) =24a(8) =45a(9) =84a(10) =160a(11) =300a(12) =565a(13) =1064a(14) =2005a(15) =3774a(16) =7108a(17) =13386a(18) =25209a(19) =47472a(20) =89401a(21) =168360a(22) =317056a(23) =597080a(24) =1124425a(25) =2117520a(26) =3987721a(27) =7509690a(28) =14142276a(29) =26632782