Number of hex trees with n edges and no branches of length 1.

Terms

a(0) =1a(1) =0a(2) =9a(3) =27a(4) =90a(5) =297a(6) =1053a(7) =3888a(8) =14742a(9) =56619a(10) =219429a(11) =857304a(12) =3375999a(13) =13391001a(14) =53452467a(15) =214525017a(16) =865041606a(17) =3502806363a(18) =14237599635a(19) =58069495188a(20) =237583710549a(21) =974819569095