a(n) = ((1 + sqrt(15))^n - (1 - sqrt(15))^n)/sqrt(15).

Terms

a(0) =0a(1) =2a(2) =4a(3) =36a(4) =128a(5) =760a(6) =3312a(7) =17264a(8) =80896a(9) =403488a(10) =1939520a(11) =9527872a(12) =46209024a(13) =225808256a(14) =1098542848a(15) =5358401280a(16) =26096402432a(17) =127210422784a(18) =619770479616