A math textbook leaving certain results as an exercise for the reader?
I think this is usually actually one of (1) the author not wanting to write out the proof (because it’s boring/tedious) or (2) a proof that would make a good exercise because it is easy enough if you understand the big ideas (and coming up with good exercises is not always easy).
I think this is usually actually one of (1) the author not wanting to write out the proof (because it’s boring/tedious) or (2) a proof that would make a good exercise because it is easy enough if you understand the big ideas (and coming up with good exercises is not always easy).