a(n) = 4*a(n-1)+3*a(n-2) for n>1, a(0)=2, a(1)=4.

Terms

a(0) =2a(1) =4a(2) =22a(3) =100a(4) =466a(5) =2164a(6) =10054a(7) =46708a(8) =216994a(9) =1008100a(10) =4683382a(11) =21757828a(12) =101081458a(13) =469599316a(14) =2181641638a(15) =10135364500a(16) =47086382914a(17) =218751625156