def mu(b, I, mu0, mu1):
    """
    Recovery rate.
    Parameters:
    -----------
    b
        hospital beds per 10,000 persons
    I
        number of infected
    mu0
        Minimum recovery rate
    mu1
        Maximum recovery rate
    """
    # recovery rate, depends on mu0, mu1, b
    mu = mu0 + (mu1 - mu0) * (b / (I + b))
    return mu