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CounterexampleConstruction: AI-generatedVerification: AI-generatedSession-authored (Fable 5 assisted)precheck passaudited 2026-08-11
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Finite strictly decreasing sequences of naturals form a tree with every finite level nonempty but no infinite branch

Statement refuted

In König's infinity lemma: an ordered finitely branching tree with a node at every level has an infinite branch, in ZF, finite branching can be replaced by arbitrary branching: every tree of finite sequences with a node at every level has an infinite branch.

Facts & Assumptions

Counterexample

technique · constructive
1.1

Let TT consist of the empty sequence and all finite strictly decreasing sequences of natural numbers. It is prefix closed. For every nn, the sequence (n1,n2,,0)(n-1,n-2,\ldots,0) is a node of length nn, so every finite level is nonempty.

construct
2.1

The root has infinitely many successors, so TT is not finitely branching. An infinite branch would be an infinite strictly decreasing sequence of naturals, but its range would have a least element by The well-ordering principle, after which the branch would have to contain a smaller one. Thus no infinite branch exists, and the missing hypothesis relative to [L1] is exactly finite branching.

step 1.1L1discharge-construct

Depends on

Used by

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Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 29 results over 15 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.