a(n) = 7*a(n-1) - 14*a(n-2) + 7*a(n-3) with a(0)=0, a(1)=1, a(2)=7.

Terms

a(0) =0a(1) =1a(2) =7a(3) =35a(4) =154a(5) =637a(6) =2548a(7) =9996a(8) =38759a(9) =149205a(10) =571781a(11) =2184910a(12) =8333871a(13) =31750824a(14) =120875944a(15) =459957169a(16) =1749692735a(17) =6654580387a(18) =25306064602a(19) =96226175941a(20) =365880389868