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CounterexampleConstruction: AI-generatedVerification: AI-generatedSession-authored (Fable 5 assisted)precheck passaudited 2026-07-31
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Two dense lobes meeting at one cut vertex give κ(G)=1<λ(G)=2<δ(G)=3\kappa(G)=1<\lambda(G)=2<\delta(G)=3

Statement refuted

The false statement FALSE: vertex connectivity, edge connectivity and minimum degree are always equal claims that vertex connectivity, edge connectivity and minimum degree always agree.

Facts & Assumptions

Given: For i=1,2i=1,2, take vertices ai,bi,ci,dia_i,b_i,c_i,d_i spanning K4K_4 with the edge aibia_ib_i deleted. Add one vertex vv, add the edges vai,vbiva_i,vb_i for both ii, and add no edge between the two four-vertex lobes.

Counterexample

technique · direct
1.1

The graph is connected, and deleting vv separates the two lobes. No deletion of zero vertices disconnects a connected graph, so κ(G)=1\kappa(G)=1.

givenF1
1.2

In each lobe, aia_i and bib_i have two neighbours inside the lobe and the neighbour vv, so degree 33; ci,dic_i,d_i have degree 33 inside the lobe; and vv has degree 44. Hence δ(G)=3\delta(G)=3.

givenF2
1.3

Deleting va1va_1 and vb1vb_1 separates the first lobe from the rest, so λ(G)2\lambda(G)\le2.

givenF1
1.4

Every edge lies on a cycle. The edges vaiva_i and vbivb_i lie on the 44-cycle v,ai,ci,bi,vv,a_i,c_i,b_i,v. Each internal edge incident with aia_i lies on the triangle ai,ci,di,aia_i,c_i,d_i,a_i, and each internal edge incident with bib_i lies on bi,ci,di,bib_i,c_i,d_i,b_i. Thus deleting one edge leaves an alternate path between its endpoints and cannot disconnect the graph, so λ(G)2\lambda(G)\ge2.

givenF1
2.1

Steps 1.3 and 1.4 give λ(G)=2\lambda(G)=2. Together with steps 1.1 and 1.2, this proves κ(G)=1<λ(G)=2<δ(G)=3\kappa(G)=1<\lambda(G)=2<\delta(G)=3 and refutes equality in both Whitney inequalities.

step 1.1step 1.2step 1.3step 1.4

Remarks

va1b1c1d1a2b2c2d2vertexcutfvgedgecutfva1;vb1g(G)=1<¸(G)=2<±(G)=3

Depends on

Used by

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Dependency tree · next 3 levels

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

Sources