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CounterexampleConstruction: AI-adaptedVerification: AI-adaptedSession-authored (Fable 5 assisted)precheck passaudited 2026-08-16
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(Z/15Z)× has order 8 but is not cyclic

Statement refuted

The order of a unit group need not be the order of one of its elements: (Z/15)× has order 8 but is not cyclic.

Facts & Assumptions

Given: The modulus 15=35.

[L1]

The unit-group structure theorem gives the product of the prime-power factors (The unit group modulo n is the product of its odd-prime cyclic factors and its explicit 2-power factor).

Counterexample

technique · direct
1.1

By [L1], (Z/15)×C2×C4, which has 8 elements.

L1algebra
2.1

For every pair (x,y)C2×C4, [L2] gives x4=1 and y4=1, so (x,y)4=(1,1). Another application of [L2] shows its order divides 4. Thus no element has order 8 and the group is not cyclic.

step 1.1L2algebra

Depends on

Used by

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Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 96 results over 23 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.

Sources