Geometric Decompositions of Div-Conforming Finite Element Tensors

A unified construction of div-conforming finite element tensors, including vector div element, symmetric div matrix element, traceless div matrix element, and in general tensors with constraints, is developed in this work. It is based on the geometric decomposition of Lagrange elements into bubble functions on each sub-simplex. Then the tensor at each sub-simplex is decomposed into the tangential and the normal component. The tangential component forms the bubble function space and the normal component characterizes the trace. A deep exploration on boundary degrees of freedom is presented for discovering various finite elements. The developed finite element spaces are div conforming and satisfy the discrete inf-sup condition. An explicit basis of the constraint tensor space is also established.