Parity arguments

Some things are impossible because an odd or even count can never change into the other. If every move flips two lights at once, the number switched on always changes by an even amount, so an odd target is unreachable. Parity proves impossibility, which counting never can.

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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 ladderNo problems written at this tier yet.Nothing written at this tier yet
Practice — 5-point ladder7 problems. Three in a row finishes it.0/3 in a row · 0/7 solved
Skill quizFinish every ladder that has problems first.Finish the ladders above first

Before and after

Needs first

LOG-01Reading a logical statement precisely NUM-03Parity of numbers (odd/even behavior)

Opens up

LOG-06Invariants and monovariants LOG-07Coloring arguments

In official papers

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