E-trees with exactly 2 colors.

Terms

a(0) =0a(1) =0a(2) =3a(3) =16a(4) =65a(5) =238a(6) =866a(7) =3128a(8) =11493a(9) =42660a(10) =160584a(11) =610640a(12) =2345504a(13) =9081768a(14) =35422175a(15) =139022622a(16) =548671517a(17) =2176051050a(18) =8668363573a(19) =34667284390a(20) =139140830956a(21) =560271494630