Ohnimus, Stephan, Marcus Rüter, and Erwin Stein. 2023. “General Aspects of Trefftz Method and Relations to Error Estimation of Finite Element Approximations”. Computer Assisted Methods in Engineering and Science 8 (2-3): 425-37. https://cames3.ippt.pan.pl/index.php/cames/article/view/1180.