2. Reference element. Notion of curved finished element

All the elements introduced in the previous paragraphs had simple geometric forms, for example, in dimension two, triangles or squares; in practice, we have to use elements with more elaborate forms, in particular any triangles or quadrilaterals or with curved sides to best approximate the $\partial \mathrm{\Omega }$ boundary, when the $\mathrm{\Omega }$ domain is not polyhedral.

The aim of this paragraph is to show how, by using certain transformations, we can obtain elements with a more complicated geometry. In addition, these transformations make it possible to calculate integrals...