a(n) = binomial(5*n, n)/(4*n + 1).

Terms

a(0) =1a(1) =1a(2) =5a(3) =35a(4) =285a(5) =2530a(6) =23751a(7) =231880a(8) =2330445a(9) =23950355a(10) =250543370a(11) =2658968130a(12) =28558343775a(13) =309831575760