a(n) = sum of absolute-valued coefficients of (1+2*x-x^2)^n.

Terms

a(0) =1a(1) =4a(2) =12a(3) =36a(4) =100a(5) =300a(6) =776a(7) =2412a(8) =6304a(9) =19036a(10) =50952a(11) =148896a(12) =393452a(13) =1211444a(14) =3167004a(15) =9672772a(16) =25295248a(17) =76084796a(18) =200590424a(19) =608621376a(20) =1617201648a(21) =4908511140a(22) =12658776540a(23) =38907904188a(24) =102775961200a(25) =310485090044