GLT closure under geometric mean

Let {An}n\{A_n\}_n and {Bn}n\{B_n\}_n be Hermitian positive definite GLT matrix-sequences with matrix-valued GLT symbols

{An}nGLTκ,{Bn}nGLTξ.\{A_n\}_n\sim_{\mathrm{GLT}}\kappa,\qquad \{B_n\}_n\sim_{\mathrm{GLT}}\xi.

Assume κ,ξ:[0,1]d×[π,π]dCr×r\kappa,\xi:[0,1]^d\times[-\pi,\pi]^d\to\mathbb{C}^{r\times r} are positive-semidefinite. For ε>0\varepsilon>0, set κε=κ+εIr\kappa_\varepsilon=\kappa+\varepsilon I_r and ξε=ξ+εIr\xi_\varepsilon=\xi+\varepsilon I_r, and define

G~(κ,ξ)(x,θ):=limε0G(κε(x,θ),ξε(x,θ))\widetilde G(\kappa,\xi)(x,\theta):=\lim_{\varepsilon\to 0}G\bigl(\kappa_\varepsilon(x,\theta),\xi_\varepsilon(x,\theta)\bigr)

for almost every (x,θ)(x,\theta). GLT closure under geometric mean. The geometric mean matrix-sequence is again GLT and

{G(An,Bn)}nGLTG~(κ,ξ).\{G(A_n,B_n)\}_n\sim_{\operatorname{GLT}}\widetilde G(\kappa,\xi).

Here GG denotes the matrix geometric mean, and G~(κ,ξ)\widetilde G(\kappa,\xi) is the candidate symbol defined by the preceding limit. This conjecture seeks to remove invertibility assumptions on the GLT symbols from spectral-distribution results for geometric means of HPD GLT sequences; its status is not resolved by the supplied text.

Sources & referencesView supporting material

Primary source

Asiim Ilyas, Muhammad Faisal Khan, Valerio Loi and Stefano Serra-Capizzano, “Geometric means of HPD GLT matrix-sequences: a maximal result beyond invertibility assumptions on the GLT symbols”, arXiv:2505.03256 (2025).

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