Kiraly–Tanigawa's body-pin rigidity characterization conjecture

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Let H=(W,E)H=(W,E) be a multigraph, and let GHG_H be the body-pin graph obtained by replacing each w∈Ww\in W with a body and identifying distinct pairs of body vertices according to the edges of HH. For disjoint subsets X,Y⊆WX,Y\subseteq W, let dH(X,Y)d_H(X,Y) be the number of edges between them and define

ℓH(X,Y)={6if dH(X,Y)≥3,5if dH(X,Y)=2,3if dH(X,Y)=1,0if dH(X,Y)=0.\ell_H(X,Y)= \begin{cases} 6 & \text{if }d_H(X,Y)\geq 3,\\ 5 & \text{if }d_H(X,Y)=2,\\ 3 & \text{if }d_H(X,Y)=1,\\ 0 & \text{if }d_H(X,Y)=0. \end{cases}

Kiraly–Tanigawa's body-pin conjecture. The graph GHG_H is rigid in R3\mathbb{R}^3 if and only if, for every partition P={P1,P2,…,Pt}\mathcal{P}=\{P_1,P_2,\dots,P_t\} of WW,

∑1≤i<j≤tℓH(Pi,Pj)≥6(∣P∣−1).\sum_{1\leq i<j\leq t}\ell_H(P_i,P_j)\geq 6(|\mathcal{P}|-1).

This is a conjectured characterization of rigidity for body-pin graphs in three dimensions. The source attributes it to Kiraly and Tanigawa and does not report a resolution.

References

Primary source

Bill Jackson, Tibor Jordán and Soma Villányi, “Rank Contributions of Vertices in Rigidity Matroids of Clique Covered Graphs”, arXiv:2607.26266 (2026).

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