a(n) = [x^n] (1 + x*(1+x)^n )^n.

Terms

a(0) =1a(1) =1a(2) =5a(3) =28a(4) =233a(5) =2376a(6) =28102a(7) =379016a(8) =5707025a(9) =94439440a(10) =1699067321a(11) =32951077193a(12) =684009742319