a(n) = 3*a(n-1) - a(n-2) - 3*a(n-3) + 2*a(n-4), where a(0) = 2, a(1) = 4, a(2) = 8, a(3) = 18.

Terms

a(0) =2a(1) =4a(2) =8a(3) =18a(4) =38a(5) =80a(6) =164a(7) =334a(8) =674a(9) =1356a(10) =2720a(11) =5450a(12) =10910a(13) =21832a(14) =43676a(15) =87366a(16) =174746a(17) =349508a(18) =699032a(19) =1398082a(20) =2796182a(21) =5592384a(22) =11184788a(23) =22369598a(24) =44739218a(25) =89478460a(26) =178956944a(27) =357913914a(28) =715827854a(29) =1431655736