def aa_or(x, y, nx, ny):
    """Dimensionless production rate for a gene regulated by two
    activators with OR logic in the absence of leakage.

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

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