Practice
The Abyss
38 problems that are hard on purpose. Some are multi-hour derivations that appear in the literature as a line reading “a standard computation shows”. Some ask you to find where a bound is loose. A few are honestly open, and are labelled as such — no solution exists to reveal, because nobody has one.
If a problem here is taking hours, that is the intended experience. Work it with the proof library open — every technique these need is proved somewhere on the site.
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