Approximation by non-self-referential bivariable fractal functions Nallapu Vijender and Vasileios Drakopoulos Eur. Phys. J. Spec. Top., 230 21-22 (2021) 3747-3753 Published online: 22 November 2021 DOI: 10.1140/epjs/s11734-021-00337-0