The number of even permutations p of 1,2,...,n such that -1<=p(i)-i<=2 for i=1,2,...,n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =7a(5) =12a(6) =22a(7) =41a(8) =75a(9) =137a(10) =252a(11) =464a(12) =853a(13) =1568a(14) =2884a(15) =5305a(16) =9757a(17) =17945a(18) =33006a(19) =60708a(20) =111659a(21) =205372a(22) =377738a(23) =694769a(24) =1277879a(25) =2350385a(26) =4323032a(27) =7951296a(28) =14624713a(29) =26899040