Kumar, D., et al. “Approximation of Entire Harmonic Functions in \(\mathbb{R}^3\) Having Index-Pair \((p,\,q)\)”. Anal. Numér. Théor. Approx., vol. 20, no. 1, Aug. 1991, pp. 47-57, https://ictp.acad.ro/jnaat/journal/article/view/1991-vol20-nos1-2-art7.