Davis, C. P. ., and K. F. . Warnick. “On the Physical Interpretation of the Sobolev Norm in Error Estimation”. Applied Computational Electromagnetics Society Journal (ACES), vol. 20, no. 2, June 2022, pp. 144-50, https://journals.riverpublishers.com/index.php/ACES/article/view/16587.