摘要：

In the equation of motion approach to the two-time Green's functions, conventional Tyablikov-type truncation of the chain of equations is rather arbitrary and apt to violate the analytical structure of Green’s functions. We propose a practical way to truncate the equations of motion using operator projection. It guarantees the causality of Green’s functions, fulfils the time translation invariance and the particle-hole symmetry, and is easy to implement in a computer. I report recent progress made in the development and application of this new method.