def single_reciprocal(x, a, b, c):
    """
    reciprocal function to fit convergence data
    """
    if isinstance(x, list):
        y_l = []
        for x_v in x:
            y_l.append(a + b / (x_v - c))
        y = np.array(y_l)
    else:
        y = a + b / (x - c)
    return y
reciprocal function to fit convergence data