Number of 5-alternating permutations.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =5a(7) =20a(8) =55a(9) =125a(10) =251a(11) =2300a(12) =15775a(13) =70500a(14) =249250a(15) =750751a(16) =10006375a(17) =97226875a(18) =601638125a(19) =2886735625a(20) =11593285251a(21) =202808749375