Second-order equations as Friedrichs systems

On the basis of the recent progress in understanding the abstract setting for Friedrichs symmetric positive systems by Ern et al. (2007) [8], as well as Antonić and Burazin (2010) [9], an attempt is made to relate these results to the classical Friedrichs theory.A particular set of sufficient conditions for a boundary matrix field to define a boundary operator is given, and the applicability of this procedure is shown by examples of boundary/initial value problems for second-order partial differential equations written as symmetric systems.