a(n) = [x^n] 2 / (1 - 7*x + sqrt(1 - 2*x - 3*x^2)).

Terms

a(0) =1a(1) =4a(2) =17a(3) =73a(4) =315a(5) =1362a(6) =5895a(7) =25528a(8) =110579a(9) =479068a(10) =2075683a(11) =8993897a(12) =38971621a(13) =168871854a(14) =731764089a(15) =3170939841a(16) =13740635787a(17) =59542470588a(18) =258016586955