a(n)=floor((2+sqrt(7))^n).

Terms

a(0) =1a(1) =4a(2) =21a(3) =100a(4) =465a(5) =2164a(6) =10053a(7) =46708a(8) =216993a(9) =1008100a(10) =4683381a(11) =21757828a(12) =101081457a(13) =469599316a(14) =2181641637a(15) =10135364500a(16) =47086382913a(17) =218751625156