  This paper presents a lowest-order immersed Raviart-Thomas mixed triangular
finite element method for solving elliptic interface problems on unfitted
meshes independent of the interface. In order to achieve the optimal
convergence rates on unfitted meshes, an immersed finite element finite (IFE)
is constructed by modifying the traditional Raviart-Thomas element. Some
important properties are derived including the unisolvence of IFE basis
functions, the optimal approximation capabilities of the IFE space and the
corresponding commuting digram. Optimal error estimates are rigorously proved
for the mixed IFE method and some numerical examples are also provided to
validate the theoretical analysis.
