a(n) = (11*5^n - 7)/4.

Terms

a(0) =1a(1) =12a(2) =67a(3) =342a(4) =1717a(5) =8592a(6) =42967a(7) =214842a(8) =1074217a(9) =5371092a(10) =26855467a(11) =134277342a(12) =671386717a(13) =3356933592a(14) =16784667967a(15) =83923339842a(16) =419616699217