a(n) = 5*4^n - 1.

Terms

a(0) =4a(1) =19a(2) =79a(3) =319a(4) =1279a(5) =5119a(6) =20479a(7) =81919a(8) =327679a(9) =1310719a(10) =5242879a(11) =20971519a(12) =83886079a(13) =335544319a(14) =1342177279a(15) =5368709119a(16) =21474836479a(17) =85899345919a(18) =343597383679