def rr_and_single(x, y, nx, ny):
    """Dimensionless production rate for a gene regulated by two
    repressors with AND logic in the absence of leakage with
    single occupancy.

    Parameters
    ----------
    x : float or NumPy array
        Concentration of first repressor.
    y : float or NumPy array
        Concentration of second repressor.
    nx : float
        Hill coefficient for first repressor.
    ny : float
        Hill coefficient for second repressor.

    Returns
    -------
    output : NumPy array or float
        1 / (1 + x**nx + y**ny)
    """
    return 1.0 / (1.0 + x ** nx + y ** ny)