We present a new method for proving Correa-Jofré-Thibault's theorem that monotonicity of subdifferential implies convexity of the function. This new method is based on barrier functions. Barrier functions help overcome some of the main technical difficulties when working with lower semicontinuous functions.

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M. Ivanov, N. Zlateva. “Barrier Functions in Subdifferential Theory.” Journal of Convex Analysis 27 (2020), No. 4, 1091–1098.