a(n) = 4^n + 6*2^n + 3^(n+1) + 10.

Terms

a(0) =20a(1) =35a(2) =77a(3) =203a(4) =605a(5) =1955a(6) =6677a(7) =23723a(8) =86765a(9) =324275a(10) =1231877a(11) =4738043a(12) =18396125a(13) =71940995a(14) =282882677a(15) =1116985163a(16) =4424500685a(17) =17568076115a(18) =69883311077a(19) =278367837083