[1]
Test.
[2]
21AD. The Collected Papers of Albert Einstein, Volume 13. (English): The Berlin Years: Writings & Correspondence, January 1922 - March 1923. Princeton University Press; Documentary edition.
[3]
13AD. The Collected Papers of Albert Einstein, Volume 14: The Berlin Years: Writings & Correspondence, April 1923-May 1925. Princeton University Press; Documentary edition.