a(0) = a(1) = 1; a(n) = 4 * Sum_{k=0..n-2} a(k) * a(n-k-2).

Terms

a(0) =1a(1) =1a(2) =4a(3) =8a(4) =36a(5) =96a(6) =416a(7) =1312a(8) =5504a(9) =19200a(10) =79168a(11) =293888a(12) =1203712a(13) =4648448a(14) =19027968a(15) =75411456a(16) =309487616a(17) =1248411648a(18) =5144133632a(19) =21011775488a(20) =86971449344a(21) =358540509184