a(n) is the number of indecomposable involutions of length n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =7a(5) =23a(6) =71a(7) =255a(8) =911a(9) =3535a(10) =13903a(11) =57663a(12) =243871a(13) =1072031a(14) =4812575a(15) =22278399a(16) =105300287a(17) =510764095a(18) =2527547455a(19) =12794891007a(20) =66012404863a(21) =347599231103