G.f. A(x) satisfies: (1 - x*A(x))^3 = 1 - 3*x - x^3*A(x^3).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =22a(6) =55a(7) =142a(8) =376a(9) =1011a(10) =2758a(11) =7614a(12) =21220a(13) =59630a(14) =168759a(15) =480533a(16) =1375676a(17) =3957075a(18) =11430582a(19) =33144264a(20) =96434321a(21) =281447954a(22) =823734157a(23) =2417092933a(24) =7109265120a(25) =20955593252a(26) =61893804180a(27) =183148075432a(28) =542885589115