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

Terms

a(0) =4a(1) =17a(2) =69a(3) =277a(4) =1109a(5) =4437a(6) =17749a(7) =70997a(8) =283989a(9) =1135957a(10) =4543829a(11) =18175317a(12) =72701269a(13) =290805077a(14) =1163220309a(15) =4652881237a(16) =18611524949a(17) =74446099797a(18) =297784399189