a(n) = 2*a(n-1) - a(n-4) for n >= 4, where a(0) = 2, a(1) = 4, a(2) = 6, a(3) = 10, a(4) = 16.

Terms

a(0) =2a(1) =4a(2) =6a(3) =10a(4) =16a(5) =28a(6) =50a(7) =90a(8) =164a(9) =300a(10) =550a(11) =1010a(12) =1856a(13) =3412a(14) =6274a(15) =11538a(16) =21220a(17) =39028a(18) =71782a(19) =132026a(20) =242832a(21) =446636a(22) =821490a(23) =1510954a(24) =2779076a(25) =5111516a(26) =9401542a(27) =17292130a(28) =31805184a(29) =58498852