The converse chordality conjecture for positive definite function extensions

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Let GG be a group, let S⊂GS\subset G, and let Γ(G,S)\Gamma(G,S) be the associated graph. A function ϕ:S→L(H)\phi:S\to {\mathcal L}({\mathcal H}) is positive definite if its corresponding kernels satisfy the positive-definiteness condition used for operator-valued functions.

Converse chordality conjecture. For every S⊂GS\subset G such that Γ(G,S)\Gamma(G,S) is not chordal, there exists a positive definite function

ϕ:S→L(H)\phi:S\to {\mathcal L}({\mathcal H})

which does admit a positive definite extension to GG.

This conjecture proposes the reciprocal of the paper's extension theorem: nonchordality should guarantee the existence of a positive definite function on SS that extends positively to the whole group. The supplied text gives no evidence that the conjecture has been resolved.

References

Primary source

M. Bakonyi and D. Timotin, “Extensions of positive definite functions on amenable groups”, arXiv:0808.1273 (2008).

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