a(n) = (85*4^n - 1)/3.

Terms

a(0) =113a(1) =453a(2) =1813a(3) =7253a(4) =29013a(5) =116053a(6) =464213a(7) =1856853a(8) =7427413a(9) =29709653a(10) =118838613a(11) =475354453a(12) =1901417813a(13) =7605671253a(14) =30422685013a(15) =121690740053a(16) =486762960213