Number of permutations of 1..n with i-6<=p(i)<=i+3.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =96a(5) =384a(6) =1536a(7) =5658a(8) =20266a(9) =72955a(10) =268381a(11) =994512a(12) =3686128a(13) =13633620a(14) =50340576a(15) =185671416a(16) =684550020a(17) =2524915648a(18) =9317169000a(19) =34387796485a(20) =126912448549a(21) =468345021626