a(n) = C(5*n+1,n).

Terms

a(0) =1a(1) =6a(2) =55a(3) =560a(4) =5985a(5) =65780a(6) =736281a(7) =8347680a(8) =95548245a(9) =1101716330a(10) =12777711870a(11) =148902215280