A traffic light problem: expansion of 2/(1 - 3*x)^3.

Terms

a(0) =2a(1) =18a(2) =108a(3) =540a(4) =2430a(5) =10206a(6) =40824a(7) =157464a(8) =590490a(9) =2165130a(10) =7794468a(11) =27634932a(12) =96722262a(13) =334807830a(14) =1147912560a(15) =3902902704a(16) =13172296626a(17) =44165935746a(18) =147219785820a(19) =488149816140