a(n) = 6^n + 4^n - 1.

Terms

a(0) =1a(1) =9a(2) =51a(3) =279a(4) =1551a(5) =8799a(6) =50751a(7) =296319a(8) =1745151a(9) =10339839a(10) =61514751a(11) =366991359a(12) =2193559551a(13) =13127802879a(14) =78632599551a(15) =471258726399