Coefficient of x^n in the expansion of ( (1-x) / (1-x-x^2) )^(2*n).

Terms

a(0) =1a(1) =0a(2) =4a(3) =6a(4) =44a(5) =120a(6) =610a(7) =2114a(8) =9468a(9) =36384a(10) =155644a(11) =626450a(12) =2638994a(13) =10856924a(14) =45565118a(15) =189579786a(16) =796023260a(17) =3333362040a(18) =14022032560a(19) =58960463548a(20) =248542728364