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

Terms

a(0) =1a(1) =1a(2) =5a(3) =19a(4) =145a(5) =851a(6) =7741a(7) =58605a(8) =600769a(9) =5420035a(10) =61026901a(11) =628076153a(12) =7648488145a(13) =87388647373