def exp_kernel(x1, x2):
    x1 = np.expand_dims(x1, 2)
    x2 = x2.T
    return np.exp(((- np.mean(np.abs((x1 - x2)), axis=1)) / L0))
def exp_kernel(x1, x2):
    x1 = np.expand_dims(x1, 2)
    x2 = x2.T
    return np.exp(((- np.mean(np.abs((x1 - x2)), axis=1)) / L0))