$\mathrm{M}$-ideals: from Banach spaces to rings
David P. Blecher, Amartya Goswami
TL;DR
The paper introduces ring $\mathrm{M}$-ideals as an algebraic analogue of Banach-space $\mathrm{M}$-ideals, showing they generalize essential ideals. It proves a central dichotomy: an ideal is a ring $\mathrm{M}$-ideal iff it is either essential or relatively irreducible, and develops stability results under intersections, quotients, direct products, and Morita equivalence. It provides explicit classifications in key settings such as $\mathbb{Z}_n$, $C(K)$, and unital $\mathrm{C}^*$-algebras, with a precise description in $\mathrm{C}^*$-algebras where nonzero $\mathrm{M}$-ideals are either essential or have prime closures. The work further connects these notions to ring structure through Morita invariance, maximal $\mathrm{M}$-ideals, and the universal existence of $\mathrm{M}$-complements, offering a rich framework linking ideal theory, module theory, and operator-algebraic contexts. These results illuminate how $\mathrm{M}$-ideals structure rings and parallel concepts like injective hulls and essential extensions in module theory, while suggesting new invariants such as $\mathrm{M}$-dimension.
Abstract
We introduce and investigate a class of ring ideals, termed ring $\mathrm{M}$-ideals, inspired by the Alfsen--Effros theory of $\mathrm{M}$-ideals in Banach spaces. We show that $\mathrm{M}$-ideals extend the classical notion of essential ideals and subsume them as a subclass. The central theorem provides a full characterization: an ideal is an $\mathrm{M}$-ideal if and only if it is either essential or relatively irreducible. This dichotomy reveals the abundant and diverse nature of $\mathrm{M}$-ideals, encompassing both essential and minimal ideals, and admits natural generalizations in rings beyond the commutative and unital settings. We systematically study the algebraic stability of $\mathrm{M}$-ideals under standard constructions such as intersection, quotient, direct product, and Morita equivalence and establish their behavior in topological rings and operator algebras. In certain rings such as $\mathbb{Z}_n$ and C*-algebras, we completely classify $\mathrm{M}$-ideals and relate them to algebraically minimal projections and central idempotents. The ring $\mathrm{M}$-ideals in $C(K)$ are shown to be precisely the essential ideals or those minimal ideals corresponding to isolated points. Structurally, we show that the absence of proper $\mathrm{M}$-ideals characterizes simplicity, while rings in which every proper $\mathrm{M}$-ideal is a direct summand must decompose as finite direct sums of simple rings. In closing, we introduce the notion of $\mathrm{M}$-complements, drawing an analogy with essential extensions in module theory, and demonstrate their existence.