a(n) = 6*a(n-1) - 9*a(n-2) + 3*a(n-3).

Terms

a(0) =0a(1) =-3a(2) =-15a(3) =-63a(4) =-252a(5) =-990a(6) =-3861a(7) =-15012a(8) =-58293a(9) =-226233a(10) =-877797a(11) =-3405564a(12) =-13211910a(13) =-51254775a(14) =-198838152a(15) =-771371667a(16) =-2992450959a(17) =-11608875207a(18) =-45035307612a(19) =-174709321686a(20) =-677764787229