Low energy extension conjecture for matrix-valued positive definite functions

Let F\mathbb{F} be the free group, let Br\mathbb{B}_r be the radius-rr ball, and let NSPD(r,d)\mathrm{NSPD}(r,d) denote the class of normalized strictly positive definite d×dd\times d matrix-valued functions on Br\mathbb{B}_r. For strictly positive definite functions on all of F\mathbb{F}, let e\mathfrak{e} denote the relative energy defined through the associated transport operators. Low energy extension conjecture. For r,dNr,d\in\mathbb{N}, ω>0\omega>0, and C1,,CnNSPD(r,d)\mathsf{C}_1,\ldots,\mathsf{C}_n\in\mathrm{NSPD}(r,d), for every m[n]m\in[n] there exists a strictly positive definite function C^m:FMatd×d(C)\widehat{\mathsf{C}}_m:\mathbb{F}\to\mathrm{Mat}_{d\times d}(\mathbb{C}) such that

Cm=C^mBr\mathsf{C}_m=\widehat{\mathsf{C}}_m\upharpoonright\mathbb{B}_r

and

e(Cm,Ck)=e(C^m,C^k)\mathfrak{e}(\mathsf{C}_m,\mathsf{C}_k)=\mathfrak{e}(\widehat{\mathsf{C}}_m,\widehat{\mathsf{C}}_k)

for all m,k[n]m,k\in[n]. The supplied context identifies this as the paper's main harmonic-analysis conjecture, but does not explain the role of ω\omega or provide evidence of resolution.

Sources & referencesView supporting material

Primary source

Peter Burton and Kate Juschenko, “Extension of positive definite functions and Connes' embedding conjecture”, arXiv:1912.12365 (2020).

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