Number of irreducible involutions of length n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =5a(5) =13a(6) =37a(7) =107a(8) =341a(9) =1141a(10) =4021a(11) =14831a(12) =57017a(13) =227617a(14) =941305a(15) =4020455a(16) =17705753a(17) =80215513a(18) =373370329a(19) =1782362219a(20) =8716939229a(21) =43615569829a(22) =223069903933