Tribonacci analog of A055502.

Terms

a(0) =0a(1) =2a(2) =3a(3) =7a(4) =13a(5) =29a(6) =53a(7) =97a(8) =181a(9) =337a(10) =617a(11) =1151a(12) =2111a(13) =3881a(14) =7151a(15) =13147a(16) =24181a(17) =44483a(18) =81817a(19) =150497a(20) =276817a(21) =509137a(22) =936469a(23) =1722431a(24) =3168097a(25) =5827001a(26) =10717561a(27) =19712669a(28) =36257237a(29) =66687469