a(n) = 2*4^n + (-1)^n*2^(n-1).

Terms

a(0) =7a(1) =34a(2) =124a(3) =520a(4) =2032a(5) =8224a(6) =32704a(7) =131200a(8) =524032a(9) =2097664a(10) =8387584a(11) =33556480a(12) =134213632a(13) =536879104a(14) =2147467264a(15) =8589967360a(16) =34359672832a(17) =137439084544a(18) =549755551744