El OuadihS. “Direct and Inverse Approximation Theorems for Functions Defined in Damek–Ricci Spaces”. Ukrains’kyi Matematychnyi Zhurnal 76, no. 8 (September 4, 2024): 1132 - 1146. Accessed September 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/7549.