A Catalan transform of (1 + 2*x)/(1 - 2*x).

Terms

a(0) =1a(1) =4a(2) =12a(3) =40a(4) =140a(5) =504a(6) =1848a(7) =6864a(8) =25740a(9) =97240a(10) =369512a(11) =1410864a(12) =5408312a(13) =20801200a(14) =80233200a(15) =310235040a(16) =1202160780a(17) =4667212440a(18) =18150270600a(19) =70690527600a(20) =275693057640