a(n) = Sum_{k=0..n} binomial(4*n-1,k).

Terms

a(0) =1a(1) =4a(2) =29a(3) =232a(4) =1941a(5) =16664a(6) =145499a(7) =1285624a(8) =11460949a(9) =102875128a(10) =928495764a(11) =8417689504a(12) =76599066579a(13) =699232769512