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CounterexampleConstruction: Literature-sourcedVerification: AI-adaptedSession-authored (Fable 5 assisted)precheck passaudited 2026-08-26
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In Z/4 the sets A=B={0,2} have A+B=2, below the Cauchy–Davenport bound 3

Statement refuted

The Cauchy-Davenport lower bound can fail for a composite modulus.

Facts & Assumptions

Given: the modulus 4 and the set A=B={0,2}Z/4.

[L1]

For prime p and nonempty A,BZ/p, Cauchy--Davenport gives A+Bmin{p,A+B1} (Cauchy–Davenport: for p prime and nonempty A,BZ/p, A+Bmin{p,A+B1}).

Counterexample

technique · direct
1.1

The four sums are 0+0=0, 0+2=2, 2+0=2 and 2+2=0, so A+B={0,2} and therefore A+B=2.

given
2.1

The Cauchy-Davenport lower bound would be min{4,2+21}=3, so the inequality fails for this composite modulus.

step 1.1
3.1

The failure occurs outside the theorem's prime-modulus hypothesis: here the modulus is 4, not a prime p, so [L1] does not apply.

L1step 2.1

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

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Sources