Symmetries in unrooted 4-trees on n+1 vertices.

Terms

a(0) =1a(1) =2a(2) =2a(3) =8a(4) =28a(5) =20a(6) =43a(7) =143a(8) =249a(9) =546a(10) =1223a(11) =2703a(12) =8107a(13) =18085a(14) =44013a(15) =114919a(16) =327712a(17) =800937a(18) =2146066a(19) =5827711a(20) =15923828a(21) =43886143a(22) =121888966a(23) =340209504a(24) =955859391a(25) =2700771322a(26) =7652412896a(27) =21784431688a(28) =62248194140a(29) =178463482459