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

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =68a(5) =232a(6) =792a(7) =2704a(8) =9232a(9) =31520a(10) =107616a(11) =367424a(12) =1254464a(13) =4283008a(14) =14623104a(15) =49926400a(16) =170459392a(17) =581984768a(18) =1987020288a(19) =6784111616a(20) =23162405888a(21) =79081400320a(22) =270000789504a(23) =921840357376