a(n) = (4*7^n+2^n)/5.

Terms

a(0) =1a(1) =6a(2) =40a(3) =276a(4) =1924a(5) =13452a(6) =94132a(7) =658860a(8) =4611892a(9) =32282988a(10) =225980404a(11) =1581861804a(12) =11073030580a(13) =77511209964a(14) =542578461556