Low energy extension conjecture for matrix-valued positive definite functions

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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,d∈Nr,d\in\mathbb{N}, ω>0\omega>0, and C1,…,Cn∈NSPD(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:F→Matd×d(C)\widehat{\mathsf{C}}_m:\mathbb{F}\to\mathrm{Mat}_{d\times d}(\mathbb{C}) such that

Cm=C^m↾Br\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.

References

Primary source

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

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