Operators approximating partial derivatives at vertices of triangulations by averaging
Let be a triangulation of a bounded polygonal domain , the space of the functions from linear on the triangles from and the interpolation operator from to . For a unit vector and an inner vertex of , we describe the set of vectors of coefficients such that the related linear combinations of the constant derivatives on the triangles surrounding are equal to for all polynomials of the total degree less than or equal to two. Then we prove that, generally, the values of the...