Let and be positive integers with . Let for and . In this paper, the authors prove (by elementatry means) that if .
This result in case was conjectured by B. Farhi [C. R., Math., Acad. Sci. Paris 341, No. 8, 469–474 (2005; Zbl 1117.11005)] and proved by S. Hong and W. Feng [C. R., Math., Acad. Sci. Paris 343, No. 11–12, 695–698 (2006; Zbl 1156.11004)]. In the same paper they also proved the above result in case .