Ghoshal, S. K., and M. Abu Masood. “Gronwall’s Vector Inequality and Its Application to a Class of Non-Self-Adjoint Linear and Non-Linear Hyperbolic Partial Differential Equations”. The Journal of the Indian Mathematical Society, vol. 38, no. 1-4, Dec. 1974, pp. 383-94, http://www.informaticsjournals.com/index.php/jims/article/view/16715.