a(n) = 6*4^n + 2.

Terms

a(0) =8a(1) =26a(2) =98a(3) =386a(4) =1538a(5) =6146a(6) =24578a(7) =98306a(8) =393218a(9) =1572866a(10) =6291458a(11) =25165826a(12) =100663298a(13) =402653186a(14) =1610612738a(15) =6442450946a(16) =25769803778a(17) =103079215106a(18) =412316860418