On q-trees. - Chong-Yun Chao; Nian-Zu Li; Shao-Ji Xu (J. Graph Theory, 1986)

Classification of chromatically unique graphs having quadratic σ-polynomials. - Nian-Zu Li; Earl Glen Whitehead Jr.; Shao-Ji Xu (J. Graph Theory, 1987)

On sigma-polynomials. - Shao-Ji Xu (Discret. Math., 1988)