Note on an inequality of F. Qi and L. Debnath. Jiang, Wei-Dong — 2009 Acta Mathematica Academiae Paedagogicae Nyí regyháziensis. New Series [electronic only]
Sharpening of Jordan's inequality and its applications. Jiang, Wei Dong; Yun, Hua — 2006 JIPAM. Journal of Inequalities in Pure & Applied Mathematics [electronic only]