a(n) = Sum_{k=0..n} T(k) where T(n) are the tribonacci numbers A000073.

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =8a(5) =15a(6) =28a(7) =52a(8) =96a(9) =177a(10) =326a(11) =600a(12) =1104a(13) =2031a(14) =3736a(15) =6872a(16) =12640a(17) =23249a(18) =42762a(19) =78652a(20) =144664a(21) =266079a(22) =489396a(23) =900140a(24) =1655616a(25) =3045153a(26) =5600910a(27) =10301680a(28) =18947744a(29) =34850335