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Exercise, Left as an

/ek-sər-ˌsīz left əz ən/

[from technical books]

Used to complete a proof 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 humor or a vast faith in the capabilities of their audiences.