G.f. is the cube of the g.f. of A006950.

Terms

a(0) =1a(1) =3a(2) =6a(3) =13a(4) =27a(5) =51a(6) =91a(7) =159a(8) =273a(9) =455a(10) =738a(11) =1179a(12) =1860a(13) =2886a(14) =4410a(15) =6667a(16) =9981a(17) =14781a(18) =21671a(19) =31512a(20) =45474a(21) =65113a(22) =92547a(23) =130689a(24) =183439a(25) =255930a(26) =355017a(27) =489895a(28) =672672a(29) =919152