a(n) is the number of (0,1) matrices A=(a_{ij}) of size n X (3n) such that each row has exactly three 1's and each column has exactly one 1 and with the restriction that no 1 stands on the diagonal from a_{11} to a_{22}.

A173789

a(n) is the number of (0,1) matrices A=(a_{ij}) of size n X (3n) such that each row has exactly three 1's and each column has exactly one 1 and with the restriction that no 1 stands on the diagonal from a_{11} to a_{22}.