Number of permutations satisfying i-3<=p(i)<=i+4, i=1..n.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =96a(5) =330a(6) =1066a(7) =3451a(8) =11581a(9) =39264a(10) =132784a(11) =446460a(12) =1497108a(13) =5023696a(14) =16878488a(15) =56739141a(16) =190697893a(17) =640763258a(18) =2152824662a(19) =7233281108a(20) =24304468132a(21) =81666680202a(22) =274410023170a(23) =922040339607