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

Terms

a(0) =4a(1) =11a(2) =19a(3) =41a(4) =79a(5) =161a(6) =319a(7) =641a(8) =1279a(9) =2561a(10) =5119a(11) =10241a(12) =20479a(13) =40961a(14) =81919a(15) =163841a(16) =327679a(17) =655361a(18) =1310719a(19) =2621441a(20) =5242879a(21) =10485761a(22) =20971519a(23) =41943041a(24) =83886079a(25) =167772161a(26) =335544319a(27) =671088641a(28) =1342177279a(29) =2684354561