1. Mitrinović, DS: Analytic Inequalities. Springer, Berlin (1970)
2. Neuman, E, Sandor, J: On some inequalities involving trigonometric and hyperbolic functions with emphasis of the Cusa-Huygens, Wilker and Huygens inequalities. Math. Inequal. Appl. 13(4), 715-723 (2010)
3. Redheffer, R: Problem 5642. Am. Math. Mon. 76, 422 (1969)
4. Kuang, J: Applied Inequalities, 3rd edn. Shandong Science and Technology Press, Jinan, Shandong (2004)
5. Chen, C-P, Cheung, W-S: Sharpness of Wilker and Huygens type inequalities. J. Inequal. Appl. 2012, 72 (2012). doi: 10.1186/1029-242x-2012-72