a(n) = binomial(4n, n) - binomial(4n, n - 3).

A026020

a(n) = binomial(4n, n) - binomial(4n, n - 3).

Terms

    a(0) =1a(1) =4a(2) =28a(3) =219a(4) =1804a(5) =15314a(6) =132572a(7) =1163565a(8) =10316924a(9) =92195488a(10) =829016968a(11) =7492106505a(12) =67991427828a(13) =619193535380

External references