exercise for the reader

From academic writing. One of the first known recorded occurrences of this phrase is in An Elementary Course of Mathematics by Thomas Stephens Davies, Stephen Fenwick, and William Rutherford from 1853.

exercise for the reader (plural not attested)

1. Used other than figuratively or idiomatically: see exercise,‎ for,‎ the,‎ reader.
2. (by extension) A matter left to the addressee's judgement to decide.
   • 2003, Wells Earl Draughon, Advanced Writing: Fiction and Film, iUniverse, →ISBN, page 167:

     She does not ask to see the dossier on this man before she goes, even though the co-worker has said that he has seen it, etc. The credibility of this conduct is another matter and is left as an exercise for the reader.

   • 2006, Hossein Bidgoli, Handbook of Information Security, Information Warfare, Social, Legal, and International Issues and Security Foundations, John Wiley & Sons, →ISBN, page 413:

     Moreover, determining what is “passionate,” “artistic,” and “suitable for young children” is left as an exercise for the reader.