Number of permutations of {1,2,...,n} in which the fixed points and the non-fixed points alternate.

Terms

a(0) =1a(1) =1a(2) =0a(3) =1a(4) =2a(5) =3a(6) =4a(7) =11a(8) =18a(9) =53a(10) =88a(11) =309a(12) =530a(13) =2119a(14) =3708a(15) =16687a(16) =29666a(17) =148329a(18) =266992a(19) =1468457a(20) =2669922a(21) =16019531a(22) =29369140a(23) =190899411a(24) =352429682a(25) =2467007773a(26) =4581585864a(27) =34361893981a(28) =64142202098a(29) =513137616783