a(n) = 2*a(n-1) - a(n-3) for n >= 5, a(0) = 2, a(1) = 4, a(2) = 7, a(3) = 12, a(4) = 22.

Terms

a(0) =2a(1) =4a(2) =7a(3) =12a(4) =22a(5) =37a(6) =62a(7) =102a(8) =167a(9) =272a(10) =442a(11) =717a(12) =1162a(13) =1882a(14) =3047a(15) =4932a(16) =7982a(17) =12917a(18) =20902a(19) =33822a(20) =54727a(21) =88552a(22) =143282a(23) =231837a(24) =375122a(25) =606962a(26) =982087a(27) =1589052a(28) =2571142a(29) =4160197