On Patched Variational Principles with Application to Elliptic and Mixed Elliptic-Hyperbolic Problems. G.J. Fix; M.E. Gurtin — 1977 Numerische Mathematik
On Mixed Finite Element Methods for First Order Elliptic Systems. R.A. Nicolaides; G.J. Fix; M.D. Gunzburger — 1981 Numerische Mathematik