a(n) = 9*4^n - 2.

Terms

a(0) =34a(1) =142a(2) =574a(3) =2302a(4) =9214a(5) =36862a(6) =147454a(7) =589822a(8) =2359294a(9) =9437182a(10) =37748734a(11) =150994942a(12) =603979774a(13) =2415919102a(14) =9663676414a(15) =38654705662a(16) =154618822654a(17) =618475290622