a(n) = 4*a(n-1) + 2*a(n-2) for n>1, a(0)=0, a(1)=1.

Terms

a(0) =0a(1) =1a(2) =4a(3) =18a(4) =80a(5) =356a(6) =1584a(7) =7048a(8) =31360a(9) =139536a(10) =620864a(11) =2762528a(12) =12291840a(13) =54692416a(14) =243353344a(15) =1082798208a(16) =4817899520a(17) =21437194496a(18) =95384577024a(19) =424412697088