Number of irreducible involutions of length n avoiding the pattern {123}.

Terms

a(0) =1a(1) =3a(2) =4a(3) =9a(4) =16a(5) =31a(6) =58a(7) =112a(8) =211a(9) =411a(10) =781a(11) =1526a(12) =2923a(13) =5721a(14) =11023a(15) =21610a(16) =41821a(17) =82112a(18) =159460a(19) =313503a(20) =610531a(21) =1201721a(22) =2345734a(23) =4621899a(24) =9039472a(25) =17827054a(26) =34923997a(27) =68930585a(28) =135231649a(29) =267104311