a(n) = a(n-1) + 2*a(n-2) - a(n-3), where a(0) = 2, a(1) = 4, a(2) = 7.

Terms

a(0) =2a(1) =4a(2) =7a(3) =13a(4) =23a(5) =42a(6) =75a(7) =136a(8) =244a(9) =441a(10) =793a(11) =1431a(12) =2576a(13) =4645a(14) =8366a(15) =15080a(16) =27167a(17) =48961a(18) =88215a(19) =158970a(20) =286439a(21) =516164a(22) =930072a(23) =1675961a(24) =3019941a(25) =5441791a(26) =9805712a(27) =17669353a(28) =31838986a(29) =57371980