Hrabova, U.Z., and I.V. Kal’chuk. “Approximation of the Classes $W^{r}_{\beta,\infty}$ by Three-Harmonic Poisson Integrals”. Carpathian Mathematical Publications, vol. 11, no. 2, Dec. 2019, pp. 321-34, doi:10.15330/cmp.11.2.321-334.