Question

which economist developed an economic theory that became influential during the global depression as well as the rest of twentieth century

Answer 1

Keynesian economics basically supported the idea of government support/intervention/stimulus as a way to support the economy during an economic depression, and these ideas were well-integrated into some domestic policies that tried to keep people in work. For example, the CCC or Civilian Conservation Corps (I think) consisted of workers across the US hired to do civil works projects in order to maybe bring more people into certain areas (economic support) while compensating them - it gave people work! :)