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

Terms

a(0) =38a(1) =146a(2) =578a(3) =2306a(4) =9218a(5) =36866a(6) =147458a(7) =589826a(8) =2359298a(9) =9437186a(10) =37748738a(11) =150994946a(12) =603979778a(13) =2415919106a(14) =9663676418a(15) =38654705666a(16) =154618822658a(17) =618475290626