a(n) = 2*a(n-1) - a(n-2) + a(n-3) - a(n-4), where a(0) = 2, a(1) = 4, a(2) = 5, a(3) = 6.

Terms

a(0) =2a(1) =4a(2) =5a(3) =6a(4) =9a(5) =13a(6) =18a(7) =26a(8) =38a(9) =55a(10) =80a(11) =117a(12) =171a(13) =250a(14) =366a(15) =536a(16) =785a(17) =1150a(18) =1685a(19) =2469a(20) =3618a(21) =5302a(22) =7770a(23) =11387a(24) =16688a(25) =24457a(26) =35843a(27) =52530a(28) =76986a(29) =112828