a(n) = floor(exp(2*n)).

Terms

a(0) =1a(1) =7a(2) =54a(3) =403a(4) =2980a(5) =22026a(6) =162754a(7) =1202604a(8) =8886110a(9) =65659969a(10) =485165195a(11) =3584912846a(12) =26489122129a(13) =195729609428