LONDON, March 31 (Reuters) - Four artificial intelligence experts have expressed concern after their work was cited in an open letter – co-signed by Elon Musk – demanding an urgent pause in research. Dan Hendrycks, director of the California-based Center for AI Safety, who was also cited in the letter, stood by its contents, telling Reuters it was sensible to consider black swan events - those which appear unlikely, but would have devastating consequences. See here for a complete list of exchanges and delays