Sum of odd-indexed terms of tribonacci numbers.

Terms

a(0) =0a(1) =1a(2) =5a(3) =18a(4) =62a(5) =211a(6) =715a(7) =2420a(8) =8188a(9) =27701a(10) =93713a(11) =317030a(12) =1072506a(13) =3628263a(14) =12274327a(15) =41523752a(16) =140473848a(17) =475219625a(18) =1607656477a(19) =5438662906a(20) =18398864822a(21) =62242913851a(22) =210566269283a(23) =712340586524