Permutation rooted trees with n nodes.

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =25a(5) =77a(6) =262a(7) =897a(8) =3208a(9) =11658a(10) =43243a(11) =162477a(12) =618219a(13) =2374699a(14) =9200541a(15) =35903017a(16) =140997527a(17) =556798525a(18) =2209685939a(19) =8807924914a(20) =35248187347a(21) =141564134395a(22) =570402287162