Real equivariant almost-commuting sphere matrices conjecture

Let X1X_1, Y1Y_1 and H1H_1 be matrices in M2n(C)\boldsymbol{M}_{2n}(\mathbb{C}) satisfying

X1=X1=X1,Y1=Y1=Y1,H1=H1=H1,X_1^{\dagger}=X_1=-\overline{X_1},\qquad Y_1^{\dagger}=Y_1=-\overline{Y_1},\qquad H_1^{\dagger}=H_1=\overline{H_1},

and

\originalleft[H1,X1]\aftergroup\originalrightδ,\originalleft[H1,Y1]\aftergroup\originalrightδ,\originalleftX12+Y12+H12I\aftergroup\originalrightδ.\mathopen{}\mathclose\bgroup\originalleft\Vert [H_1,X_1]\aftergroup\egroup\originalright\Vert\leq\delta,\qquad \mathopen{}\mathclose\bgroup\originalleft\Vert [H_1,Y_1]\aftergroup\egroup\originalright\Vert\leq\delta,\qquad \mathopen{}\mathclose\bgroup\originalleft\Vert X_1^2+Y_1^2+H_1^2-I\aftergroup\egroup\originalright\Vert\leq\delta.

Real equivariant almost-commuting sphere matrices conjecture. For every η>0\eta>0 there is a δ>0\delta>0, independent of nn, such that there is a triple (X2,Y2,H2)(X_2,Y_2,H_2) of commuting Hermitian matrices satisfying

X2ρ=X2,Y2ρ=Y2,H2ρ=H2,X_2^{\rho}=-X_2,\qquad Y_2^{\rho}=-Y_2,\qquad H_2^{\rho}=H_2,

and

X22+Y22+H22=I,X_2^2+Y_2^2+H_2^2=I,

and lying within η\eta of the original triple:

\originalleftX2X1\aftergroup\originalrightη,\originalleftY2Y1\aftergroup\originalrightη,\originalleftH2H1\aftergroup\originalrightη.\mathopen{}\mathclose\bgroup\originalleft\Vert X_2-X_1\aftergroup\egroup\originalright\Vert\leq\eta,\qquad \mathopen{}\mathclose\bgroup\originalleft\Vert Y_2-Y_1\aftergroup\egroup\originalright\Vert\leq\eta,\qquad \mathopen{}\mathclose\bgroup\originalleft\Vert H_2-H_1\aftergroup\egroup\originalright\Vert\leq\eta.

This asserts stable approximation of the real-symmetry-constrained approximate sphere relations by exactly commuting matrices. It is motivated by the analogous result for almost commuting real orthogonal matrices and would yield the corresponding classification result for the rotation-equivariant two-sphere setting if true.

Sources & referencesView supporting material

Primary source

Ki Young Lee, Stephan Wong, Sachin Vaidya, Terry A. Loring and Alexander Cerjan, “Classification of Fragile Topology Enabled by Matrix Homotopy”, arXiv:2503.03948 (2025).

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