ELFT HÍRADÓ

[tcsaba <tcsaba AT eik.bme.hu>]

                 M E G H Í V Ó - I N V I T A T I O N

    Seminar Series of the Department of Theoretical Physics at the
           Budapest University of Technology and Economics


                          Péter Surján
              (ELTE Department of Physical Chemistry)

        The problem of convergence in perturbation theory


Though the Rayleigh-Schrödinger perturbation theory (PT) is a very old 
issue, the necessary and sufficient conditions of the convergence of 
this series are still not known. This is not only of theoretical 
interest, since the perturbational expansion of the correlation energy 
in many-body perturbation theory is often divergent in practice.
In this seminar, we investigate two issues:
1) Is it possible to convert a divergent PT to a convergent one by 
redefining the partitioning, that is by redefining the zero order 
Hamiltonian?
2) Knowing the members of the divergent series, is it possible to set up 
a mathematical tool to estimate the exact physical result from the 
partial sums?
As to question 2), a classical affirmative answer is to use Pade 
approximants for resummation. However, we have found that they are not 
very useful in practice. Instead, we investigate the possibility to 
scale down the divergent contributions, making them convergent thereby, 
and try to find analytical continuation of these convergent results to 
the physical situation with no scaling.


Időpont: 2017. december 8. péntek, 10:25
Helyszín: BME Fizikai Intézet, Elméleti Fizika Tanszék,
Budafoki út 8. F-épület, III lépcsőház, szemináriumi szoba