a(n) = (3^3)*4^(n-3) with a(0)=1, a(1)=1 and a(2)=7.

Terms

a(0) =1a(1) =1a(2) =7a(3) =27a(4) =108a(5) =432a(6) =1728a(7) =6912a(8) =27648a(9) =110592a(10) =442368a(11) =1769472a(12) =7077888a(13) =28311552a(14) =113246208a(15) =452984832a(16) =1811939328a(17) =7247757312a(18) =28991029248a(19) =115964116992a(20) =463856467968