Number of permutations of 1..n with i-9<=p(i)<=i+2.

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =54a(5) =162a(6) =486a(7) =1458a(8) =4374a(9) =13122a(10) =38854a(11) =115027a(12) =342013a(13) =1017864a(14) =3029092a(15) =9013876a(16) =26821828a(17) =79807684a(18) =237456052a(19) =706492756a(20) =2101954468a(21) =6253701604a(22) =18606251668a(23) =55358626421a(24) =164706941845a(25) =490047239090