Source : Free On-Line Dictionary of Computing

exercise, left as an
     
        Used to complete a proof in technical books when one doesn't
        mind a {handwave}, or to avoid one entirely.  The complete
        phrase is: "The proof [or "the rest"] is left as an exercise
        for the reader."  This comment *has* occasionally been
        attached to unsolved research problems by authors possessed of
        either an evil sense of humour or a vast faith in the
        capabilities of their audiences.
     
        [{Jargon File}]
     
        (1995-02-20)