a(n) = binomial(5*n,n)/5.

Terms

a(0) =1a(1) =9a(2) =91a(3) =969a(4) =10626a(5) =118755a(6) =1344904a(7) =15380937a(8) =177232627a(9) =2054455634a(10) =23930713170a(11) =279871768995