Pachpatte, B. G. “On a Nonlinear Integral Inequality Arising in the Theory of Differential Equations”. Rev. Anal. Numér. Théor. Approx., vol. 25, no. 1, Aug. 1996, pp. 173-8, https://ictp.acad.ro/jnaat/journal/article/view/1996-vol25-nos1-2-art17.