The extremal principle

Look at the largest, the smallest, or the leftmost object, because the extreme one often has no room to misbehave. Arguments that go nowhere in general frequently collapse in one line once you consider the extreme case.

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LearnWhat it is, how to spot it, a worked example, and where it goes wrong.
Practice — 3-point ladderNo problems written at this tier yet.Nothing written at this tier yet
Practice — 4-point ladder1 problem. Three in a row finishes it.0/3 in a row · 0/1 solved
Practice — 5-point ladder3 problems. Three in a row finishes it.0/3 in a row · 0/3 solved
Skill quizFinish every ladder that has problems first.Finish the ladders above first

Before and after

Needs first

LOG-09Informal proof by contradiction

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Nothing yet. No other skill in the Atlas lists this one as a prerequisite.

In official papers

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