  Derivation of a quantum master equation for a system weakly coupled to a bath
which takes into account nonsecular effects, but nevertheless has the
mathematically correct Gorini-Kossakowski-Lindblad-Sudarshan form (in
particular, it preserves positivity of the density operator) and also satisfies
the standard thermodynamic properties is a known long-standing problem in
theory of open quantum systems. The nonsecular terms are important when some
energy levels of the system or their differences (Bohr frequencies) are nearly
degenerate. We provide a fully rigorous derivation of such equation based on a
formalization of the weak-coupling limit for the general case.
