College of Mathematics and Informatics, Henan Polytechnic University, Jiaozuo City, Henan 454010, China, E-mail: chenchaoping@hpu.edu.cn, chenchaoping@sohu.com
(Communicated by Toka Diagana)
For all positive integers n,
Both bounds are the best possible.
Wallis’ inequality, gamma function, monotonicity
