2nd power of rooted tree enumerator; number of linear forests of 2 rooted trees.

Terms

a(0) =1a(1) =2a(2) =5a(3) =12a(4) =30a(5) =74a(6) =188a(7) =478a(8) =1235a(9) =3214a(10) =8450a(11) =22370a(12) =59676a(13) =160140a(14) =432237a(15) =1172436a(16) =3194870a(17) =8741442a(18) =24007045a(19) =66154654a(20) =182864692a(21) =506909562a(22) =1408854940a(23) =3925075510a(24) =10959698606a(25) =30665337738a(26) =85967279447a(27) =241433975446a(28) =679192039401