Sum of even-indexed terms of tribonacci numbers.

Terms

a(0) =0a(1) =1a(2) =3a(3) =10a(4) =34a(5) =115a(6) =389a(7) =1316a(8) =4452a(9) =15061a(10) =50951a(11) =172366a(12) =583110a(13) =1972647a(14) =6673417a(15) =22576008a(16) =76374088a(17) =258371689a(18) =874065163a(19) =2956941266a(20) =10003260650a(21) =33840788379a(22) =114482567053a(23) =387291750188