For n = 0, 1, 2, a(n) = n; thereafter, a(n) = 2*a(n-1) - a(n-2) + a(n-3).

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =5a(5) =9a(6) =16a(7) =28a(8) =49a(9) =86a(10) =151a(11) =265a(12) =465a(13) =816a(14) =1432a(15) =2513a(16) =4410a(17) =7739a(18) =13581a(19) =23833a(20) =41824a(21) =73396a(22) =128801a(23) =226030a(24) =396655a(25) =696081a(26) =1221537a(27) =2143648a(28) =3761840a(29) =6601569