Here is a comprehensive introduction to the least-squares finite element
method (LSFEM) for numerical solution of PDEs. It covers the theory for
first-order systems, particularly the div-curl and the div-curl-grad
system. Then LSFEM is applied systematically to permissible boundary
conditions for the incompressible Navier-Stokes equations, to show that
the divergence equations in the Maxwell equations are not redundant, and
to derive equivalent second-order versions of the Navier-Stokes
equations and the Maxwell equations. LSFEM is simple, efficient and
robust, and can solve a wide range of problems in fluid dynamics and
electromagnetics, including incompressible viscous flows, rotational
inviscid flows, low-Mach-number compressible flows, two-fluid and
convective flows, scattering waves, etc.