a(n) = 27*2^n - 5.

Terms

a(0) =22a(1) =49a(2) =103a(3) =211a(4) =427a(5) =859a(6) =1723a(7) =3451a(8) =6907a(9) =13819a(10) =27643a(11) =55291a(12) =110587a(13) =221179a(14) =442363a(15) =884731a(16) =1769467a(17) =3538939a(18) =7077883a(19) =14155771a(20) =28311547a(21) =56623099a(22) =113246203a(23) =226492411a(24) =452984827a(25) =905969659a(26) =1811939323a(27) =3623878651a(28) =7247757307a(29) =14495514619