# In memoriam Leonid V. Keldysh

## TL;DR

This paper memorializes Leonid Keldysh, highlighting his groundbreaking contributions to theoretical physics, including real-time Green functions and strong field ionization, which significantly advanced the field and inspired experiments.

## Contribution

It provides a comprehensive review of Keldysh's diverse theoretical developments, emphasizing his influential theories and their impact on physics.

## Key findings

- Keldysh formulated the diagrammatic approach to nonequilibrium Green functions.
- He developed the theory of strong field ionization of atoms.
- His work stimulated numerous experimental studies in physics.

## Abstract

Leonid Keldysh -- one of the most influential theoretical physicists of the 20th century -- passed away in November 2016. Keldysh is best known for the diagrammatic formulation of real-time (nonequilibrium) Green functions theory and for the theory of strong field ionization of atoms. Both theories profoundly changed large areas of theoretical physics and stimulated important experiments. Both these discoveries emerged almost simultaneously -- like Einstein, also Keldysh had his \textit{annus mirabilis} -- the year 1964. But the list of his theoretical developments is much broader and is briefly reviewed here.