English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

Starting from the earlier notions of stationary action principles, we showhow Schwinger's Quantum Action Principle descended from Dirac's formulation,which independently led Feynman to his path-integral formulation of quantummechanics. The connection between the two is brought out, and applications arediscussed. The Keldysh-Schwinger time-cycle method of extracting matrixelements is described. Part II will discuss the variational formulation ofquantum electrodynamics and the development of source theory.

Schwinger's Quantum Action Principle. I. From Dirac's Formulation through Feynman's Path Integrals to the Schwinger-Keldysh Method