  When an Approximation Theorist looks at well-posed PDE problems or operator
equations, and standard solution algorithms like Finite Elements, Rayleigh-Ritz
or Trefftz techniques, methods of fundamental or particular solutions and their
combinations, they boil down to approximation problems and stability issues.
These two can be handled by Approximation Theory, and this paper shows how,
with special applications to the aforementioned algorithms. The intention is
that the Approximation Theorists viewpoint is helpful for readers who are
somewhat away from that subject.
