A FREENESS CRITERION WITHOUT PATCHING FOR MODULES OVER LOCAL RINGS

It is proved that if $\varphi \colon A\to B$ is a local homomorphism of commutative noetherian local rings, a nonzero finitely generated B-module N whose flat dimension over A is at most $\operatorname {edim} A - \operatorname {edim} B$ is free over B and $\varphi $ is a special type of complete intersection. This result is motivated by a ‘patching method’ developed by Taylor and Wiles and a conjecture of de Smit, proved by the first author, dealing with the special case when N is flat over A.

CANONICAL AND -CANONICAL MODULES OF A NOETHERIAN ALGEBRA

We define canonical and $n$-canonical modules of a module-finite algebra over a Noether commutative ring and study their basic properties. Using $n$-canonical modules, we generalize a theorem on $(n,C)$-syzygy by Araya and Iima which generalize a well-known theorem on syzygies by Evans and Griffith. Among others, we prove a noncommutative version of Aoyama’s theorem which states that a canonical module descends with respect to a flat local homomorphism.

1-, 2-, AND 6-QUBITS, AND THE RAMANUJAN–NAGELL THEOREM

A conjecture of Ramanujan that was later proved by Nagell is used to show on the basis of matching dimensions that only three n-qubit systems, for n = 1, 2, 6, can possibly share an isomorphism of their symmetry algebras with those of rotations in corresponding dimensions 3, 6, 91. Such isomorphisms are valuable for use in quantum information. Simple algebraic analysis, however, already rules out the last case so that one and two qubits are the only instances of such isomorphism of the algebras and of a local homomorphism of the corresponding symmetry groups. A more mathematical topological analysis of the group spaces is also provided demonstrating their topological inequivalence.

Filter Rings Under Flat Base Change

Let φ: (R, 𝔪) → (S, 𝔫) be a flat local homomorphism of rings. In this paper, we prove: (1) If dim S/𝔪S > 0, then S is a filter ring if and only if R and k(𝔭) ⊗R𝔭 S𝔮 are Cohen–Macaulay for all 𝔮 ∈ Spec (S) \ {𝔫} and 𝔭= 𝔮 ∩ R, and S/𝔭S is catenary and equidimensional for all minimal prime ideals 𝔭 of R. (2) If dim S/𝔪S = 0, then S is a filter ring if and only if R is a filter ring and k(𝔭) ⊗R𝔭 S𝔮 is Cohen–Macaulay for all 𝔮 ∈ Spec (S) \ {𝔫} and 𝔭 = 𝔮 ∩ R, and S/𝔭S is catenary and equidimensional for all minimal prime ideals 𝔭 of R. As an application, it is shown that for a k-algebra R and an algebraic field extension K of k, if K ⊗k R is locally equidimensional, then R is a locally filter ring if and only if K ⊗k R is a locally filter ring.

Local Homomorphisms of Topological Groups

A mapping f : G → s from a left topological group G into a semigroup S is a local homomorphism if for every x є G \ {e}, there is a neighborhood Ux of e such that f (xy) = f (x)f (y) for all y є Ux \ {e}. A local homomorphism f : G → S is onto if for every neighborhood U of e, f(U \ {e}) = S. We show that(1) every countable regular left topological group containing a discrete subset with exactly one accumulation point admits a local homomorphism onto N,(2) it is consistent that every countable topological group containing a discrete subset with exactly one accumulation point admits a local homomorphism onto any countable semigroup,(3) it is consistent that every countable nondiscrete maximally almost periodic topological group admits a local homomorphism onto the countably infinite right zero semigroup.

REFLECTION OF SOME QUASI-LOCAL DOMAINS

If (R, M) and (S, N) are quasi-local domains and f : R → S is a ring homomorphism, then f is said to be a strong local homomorphism if f(M) = N. Let PVD be the category whose objects are the pseudo-valuation domains that are not fields and whose morphisms are the strong local homomorphisms; let VD be the full subcategory of PVD whose objects are all the valuation domains that are not fields. Then VD is shown to be a reflective subcategory of PVD. This result is extended by obtaining a reflective conclusion for a category whose class of objects properly contains all pseudo-valuation domains.

Universal mapping properties of some pseudovaluation domains and related quasilocal domains

If(R,M)and(S,N)are quasilocal (commutative integral) domains andf:R→Sis a (unital) ring homomorphism, thenfis said to be astrong local homomorphism(resp.,radical local homomorphism) iff(M)=N(resp.,f(M)⊆Nand for eachx∈N, there exists a positive integertsuch thatxt∈f(M)). It is known that iff:R→Sis a strong local homomorphism whereRis a pseudovaluation domain that is not a field andSis a valuation domain that is not a field, thenffactors via a unique strong local homomorphism through the inclusion mapiRfromRto its canonically associated valuation overring(M:M). Analogues of this result are obtained which delete the conditions thatRandSare not fields, thus obtaining new characterizations of wheniRis integral or radicial. Further analogues are obtained in which the “pseudovaluation domain that is not a field” condition is replaced by the APVDs of Badawi-Houston and the “strong local homomorphism” conditions are replaced by “radical local homomorphism.”