a(n) = 4*a(n-1) - a(n-2) with a(0) = 0, a(1) = 1.

Terms

a(0) =0a(1) =1a(2) =4a(3) =15a(4) =56a(5) =209a(6) =780a(7) =2911a(8) =10864a(9) =40545a(10) =151316a(11) =564719a(12) =2107560a(13) =7865521a(14) =29354524a(15) =109552575a(16) =408855776a(17) =1525870529a(18) =5694626340a(19) =21252634831a(20) =79315912984a(21) =296011017105