a(n) = 5*2^n - 1.

Terms

a(0) =4a(1) =9a(2) =19a(3) =39a(4) =79a(5) =159a(6) =319a(7) =639a(8) =1279a(9) =2559a(10) =5119a(11) =10239a(12) =20479a(13) =40959a(14) =81919a(15) =163839a(16) =327679a(17) =655359a(18) =1310719a(19) =2621439a(20) =5242879a(21) =10485759a(22) =20971519a(23) =41943039a(24) =83886079a(25) =167772159a(26) =335544319a(27) =671088639a(28) =1342177279a(29) =2684354559