Posted by Left as an exercise for the reader on November 23, 2010 at 09:52

Origin of "Left as an exercise for the reader."

There are teachers and authors, usually of textbooks in mathematics, who use expressions along the lines of "The proof is left as an exercise for the reader." Here's a web page on "exercise for the reader": http://www.everything2.com/index.pl?node=exercise%20for%20the%20reader

So: who started this expression?