a(n) = (3*7^n+(-1)^n)/4.

Terms

a(0) =1a(1) =5a(2) =37a(3) =257a(4) =1801a(5) =12605a(6) =88237a(7) =617657a(8) =4323601a(9) =30265205a(10) =211856437a(11) =1482995057a(12) =10380965401a(13) =72666757805a(14) =508667304637