MURTHY, K. N., and C. V. RAO. 1998. “THEORY OF DIFFERENTIAL INEQUALITIES ASSOCIATED WITH $n$th ORDER NONLINEAR DIFFERENTIAL EQUATIONS AND THEIR APPLICATIONS TO THREE POINT BOUNDARY VALUE PROBLEMS”. Tamkang Journal of Mathematics 29 (2):137-44. https://doi.org/10.5556/j.tkjm.29.1998.4285.