a(n) = sum of absolute-valued coefficients of (1+3*x-x^2)^n.

Terms

a(0) =1a(1) =5a(2) =21a(3) =77a(4) =291a(5) =1119a(6) =3523a(7) =15007a(8) =50923a(9) =182669a(10) =701121a(11) =2379129a(12) =8909361a(13) =32490021a(14) =106309861a(15) =423990203a(16) =1456199483a(17) =5089398187a(18) =19942506259a(19) =65753622619a(20) =252337832801a(21) =903751067081