Let oK be the integral closure of Zp in a finite field extension K of Qp, and let F be a one-dimensional full formal group defined over oK . We study certain finite subgroups C of F and prove a conjecture of Jonathan Lubin concerning the absolute endomorphism ring of the quotient F/C when F has height 2. We also investigate ways in which this result can be generalized to p-adic formal groups of...

Generalizing a method of Sutherland and the author for elliptic curves [5, 1], we design a subexponential algorithm for computing the endomorphism ring structure of ordinary abelian varieties of dimension two over finite fields. Although its correctness and complexity bound rely on several assumptions, we report on practical computations showing that it performs very well and can easily handle ...

It is proved that a commutative ring is clean if and only if it is Gelfand with a totally disconnected maximal spectrum. It is shown that each indecomposable module over a commutative ring R satisfies a finite condition if and only if R P is an artinian valuation ring for each maximal prime ideal P. Commutative rings for which each indecomposable module has a local endomorphism ring are studied...

For a ring endomorphism α and an α-derivation δ, we introduce α-compatible ideals which are a generalization of α-rigid ideals and study the connections of the prime radical and the upper nil radical of R with the prime radical and the upper nil radical of the Ore extension R[x;α, δ] and the skew power series R[[x;α]]. As a consequence we obtain a generalization of Hong, Kwak and Rizvi, 2005.

We show that the quotients of the continuous cluster category Cπ and the rational subcategory X modulo the additive subcategory generated by any cluster are abelian categories and we show that they are isomorphic to categories of infinite and finite length modules, respectively, over the endomorphism ring of the cluster. These theorems extend the theorems of Caldero-ChapotonSchiffler and Buan-M...

We determine the character table of the endomorphism ring of the permutation module associated with the multiplicity-free action of the sporadic simple Baby Monster group B on its conjugacy class 2B, where the centraliser of a 2B-element is a maximal subgroup of shape 2.Co2. This is one of the first applications of a new general computational technique to enumerate big orbits. Mathematics Subje...

We present a new method to evaluate large degree isogenies between elliptic curves over finite fields. Previous approaches all have exponential running time in the logarithm of the degree. If the endomorphism ring of the elliptic curve is ‘small’ we can do much better, and we present an algorithm with a running time that is polynomial in the logarithm of the degree. We give several applications...

The paper gives a new proof that the model categories of stable modules for the rings Z/p2 and Z/p[ ]/( 2) are not Quillen equivalent. The proof uses homotopy endomorphism ring spectra. Our considerations lead to an example of two differential graded algebras which are derived equivalent but whose associated model categories of modules are not Quillen equivalent. As a bonus, we also obtain deri...

We answer a long-standing open question by proving in ordinary set theory, ZFC, that the Kaplansky test problems have negative answers for א1-separable abelian groups of cardinality א1. In fact, there is an א1-separable abelian group M such that M is isomorphic to M ⊕M ⊕M but not to M ⊕M . We also derive some relevant information about the endomorphism ring of M .

In this paper, we give a ‘direct’ construction of the endomorphism ring of supersingular elliptic curves over a prime field Fp from ‘ideal classes’ of Q( √−p). We use the result to prove that the result of Kaneko on ‘minimal’ CM liftings of such supersingular elliptic curves is a best possible result. We also prove that the result of Elkies on ‘minimal’ CM liftings of all supersingular elliptic...