A correct version of Lemma 2.3 and Theorem 2.4 in Isoperimetric Inequalities for the Principal Eigenvalue of a Membrane and the Energy of Problems with Robin Boundary Conditions [J. Convex Analysis 22(3) (2015) 627--640] is presented.

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C. Bandle, A. Wagner. “Erratum to: Isoperimetric Inequalities for the Principal Eigenvalue of a Membrane and the Energy of Problems with Robin Boundary Conditions.” Journal of Convex Analysis 26 (2019), No. 3, 903–909.