a(n+1) = 4*(a(n)+a(n-1)) for n>1, a(1)=1, a(2)=6.

Terms

a(0) =0a(1) =1a(2) =6a(3) =28a(4) =136a(5) =656a(6) =3168a(7) =15296a(8) =73856a(9) =356608a(10) =1721856a(11) =8313856a(12) =40142848a(13) =193826816a(14) =935878656a(15) =4518821888a(16) =21818802176a(17) =105350496256a(18) =508677193728