a(n) = ((8^n) - (-6)^n)/14.

Terms

a(0) =0a(1) =1a(2) =2a(3) =52a(4) =200a(5) =2896a(6) =15392a(7) =169792a(8) =1078400a(9) =10306816a(10) =72376832a(11) =639480832a(12) =4753049600a(13) =40201179136a(14) =308548739072