'Test that convolving N(a, b) with N(c, d) gives N(a + c, b + d),
where N(., .) stands for Gaussian probability density function,
in which a and c are their means and b and d are their variances.'

@pytest.mark.parametrize(u'mode', [u'convolve_fft', u'convolve'])
def test_sum_of_gaussians(self, mode):

    kernel = models.Gaussian1D((1 / math.sqrt((2 * np.pi))), 1, 1)
    model = models.Gaussian1D((1 / math.sqrt((2 * np.pi))), 3, 1)
    model_conv = convolve_models(model, kernel, mode=mode, normalize_kernel=False)
    ans = models.Gaussian1D((1 / (2 * math.sqrt(np.pi))), 4, np.sqrt(2))
    x = np.arange((-5), 6)
    assert_allclose(ans(x), model_conv(x), atol=0.001)
