a(n) = 3*5^n - 1 = 2*A057651(n).

Terms

a(0) =2a(1) =14a(2) =74a(3) =374a(4) =1874a(5) =9374a(6) =46874a(7) =234374a(8) =1171874a(9) =5859374a(10) =29296874a(11) =146484374a(12) =732421874a(13) =3662109374a(14) =18310546874a(15) =91552734374a(16) =457763671874