The number of 5-alternating permutations of [n].

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =4a(6) =5a(7) =29a(8) =133a(9) =412a(10) =1041a(11) =2300a(12) =22991a(13) =170832a(14) =822198a(15) =3114489a(16) =10006375a(17) =141705439a(18) =1457872978a(19) =9522474417a(20) =48094772656a(21) =202808749375