a(n) = (4^n + 4^[ n/2 ] )/2.

Terms

a(0) =10a(1) =34a(2) =136a(3) =520a(4) =2080a(5) =8224a(6) =32896a(7) =131200a(8) =524800a(9) =2097664a(10) =8390656a(11) =33556480a(12) =134225920a(13) =536879104a(14) =2147516416a(15) =8589967360a(16) =34359869440a(17) =137439084544a(18) =549756338176