Coefficient of x^n in (1+n*x+x^2)^n.

Terms

a(0) =1a(1) =1a(2) =6a(3) =45a(4) =454a(5) =5775a(6) =88796a(7) =1602447a(8) =33213510a(9) =777665691a(10) =20302315252a(11) =584774029983