The converse chordality conjecture for positive definite function extensions

Let GG be a group, let SGS\subset G, and let Γ(G,S)\Gamma(G,S) be the associated graph. A function ϕ:SL(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 SGS\subset G such that Γ(G,S)\Gamma(G,S) is not chordal, there exists a positive definite function

ϕ:SL(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.

Sources & referencesView supporting material

Primary source

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

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