Hrabova, U. and Kal’chuk, I. 2019. Approximation of the classes $W^{r}_{\beta,\infty}$ by three-harmonic Poisson integrals. Carpathian Mathematical Publications. 11, 2 (Dec. 2019), 321–334. DOI:https://doi.org/10.15330/cmp.11.2.321-334.