2 Faculty of Mathemaics, Otto-von-Guericke University Magdeburg, PSF 4120, 39016 Magdeburg, Germany * Author to whom correspondence should be addressed. Mathematics 2021, 9(6), 633; https://doi.org/10.3390/math9060633 Submission received: 6 January 2021 / Revised: 22 February 2021 / Accepted...