Melenk, J. M., and I. Babuška. “Approximation With Harmonic and Generalized Harmonic Polynomials in the Partition of Unity Method”. Computer Assisted Methods in Engineering and Science, vol. 4, no. 3-4, June 2023, pp. 607-32, https://cames3.ippt.pan.pl/index.php/cames/article/view/1394.