Rakočević's conjecture on continuity of the Moore–Penrose inverse

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Let AA be a unital Banach algebra, and let AMPA_{\mathrm{MP}} denote the set of Moore–Penrose invertible elements of AA. For a∈AMPa\in A_{\mathrm{MP}}, write a†a^\dag for its Moore–Penrose inverse. Let (an)n∈N(a_n)_{n\in\mathbb{N}} be a sequence in AMPA_{\mathrm{MP}} converging to a∈AMPa\in A_{\mathrm{MP}}. Rakočević's conjecture. Then

an†→a†if and only ifsup⁡n∈N∥an†∥<∞.a_n^\dag\to a^\dag \quad\text{if and only if}\quad \sup_{n\in\mathbb{N}}\lVert a_n^\dag\rVert<\infty.

This conjecture concerns norm continuity of the Moore–Penrose inverse in unital Banach algebras. The source attributes it to Rakočević and presents the stated result as being established for certain Banach algebras, including all LpL^p-operator algebras; its status as a conjecture in the full generality stated here is not resolved by the supplied context.

References

Primary source

Eusebio Gardella, Mathias Palmstrøm and Hannes Thiel, “Rigidity of pseudofunction algebras of ample groupoids”, arXiv:2506.09563 (2025).

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