Reversion of y - y^2 - y^3 + y^4.

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =9a(5) =32a(6) =119a(7) =466a(8) =1881a(9) =7788a(10) =32868a(11) =140907a(12) =611871a(13) =2685732a(14) =11896906a(15) =53115412a(16) =238767737a(17) =1079780412a(18) =4909067468a(19) =22424085244a(20) =102865595140a(21) =473678981820