a(n) = 5*2^n - 2*n - 4.

Terms

a(0) =1a(1) =4a(2) =12a(3) =30a(4) =68a(5) =146a(6) =304a(7) =622a(8) =1260a(9) =2538a(10) =5096a(11) =10214a(12) =20452a(13) =40930a(14) =81888a(15) =163806a(16) =327644a(17) =655322a(18) =1310680a(19) =2621398a(20) =5242836a(21) =10485714a(22) =20971472a(23) =41942990a(24) =83886028a(25) =167772106a(26) =335544264a(27) =671088582a(28) =1342177220a(29) =2684354498