arXiv subjects
Publications and source records attributed to Matthias Kuenzer.
We show certain standard constructions of the theory of Verdier triangulated categories to be valid in the Heller triangulated framework as well; viz. Karoubi hull, exactness of adjoints, localisation.
Given an abelian category A with enough projectives, we can form its stable category _A_ := A/Proj(A)$. The Heller operator Omega : _A_ -> _A_ is characterised on an object X by a choice of a short exact sequence Omega X -> P -> X in A with P projective. If A is Frobenius, then Omega is an equivalence, hence has a left and a right adjoint. If A is hereditary, then Omega is zero, hence has a left and a right adjoint. In general, Omega is neither an equivalence nor zero. In the examples we have calculated via Magma, it has a left adjoint, but in general not a right adjoint. If A has projective covers, then Omega preserves monomorphisms; this would also follow from Omega having a left adjoint. I do not know an example where Omega does not have a left adjoint.
Let E be a Frobenius category, let_E_ denote its stable category. The shift functor on_E_ induces a first shift functor on the category of acyclic complexes with entries in_E_ by pointwise application. Shifting a complex by 3 positions yields a second shift functor on this category. Passing to the quotient modulo split acyclic complexes, Heller remarked that these two shift functors become isomorphic, via an isomorphism satisfying still a further compatibility. Moreover, Heller remarked that a choice of such an isomorphism determines a triangulation on_E_, except for the octahedral axiom. We generalize the notion of acyclic complexes such that the accordingly enlarged version of Heller's construction includes octahedra.
Suppose given functors A x A' -F-> B -G-> C between abelian categories, an object X in A and an object X' in A' such that certain conditions hold. We show that, E_1-terms exempt, the Grothendieck spectral sequence of the composition of F(X,-) and G evaluated at X' is isomorphic to the Grothendieck spectral sequence of the composition of F(-,X') and G evaluated at X. So instead of "resolving X' twice", we may just as well "resolve X twice".
We show by an example that in a Verdier triangulated category, there may exist two mutually nonisomorphic Verdier octahedra containing the same commutative triangle.
Let p be an odd prime. We consider the cyclotomic extension T := Z_(p)[zeta_{p^2}] of S := Z_(p), with galois group G := (Z/p^2)^*. Since this extension is wildly ramified, the SG-module T is not projective. We calculate its cohomology ring H^*(G, T; S), carrying the cup product induced by the ring structure of T. Proceeding in a somewhat greater generality, our results also apply to certain Lubin-Tate extensions.
Suppose given a commutative quadrangle in a Verdier triangulated category such that there exists an induced isomorphism on the horizontally taken cones. Suppose that the endomorphism ring of the initial or the terminal corner object of this quadrangle satisfies a finiteness condition. Then this quadrangle is homotopy cartesian.
Suppose given a Frobenius category E, i.e. an exact category with a big enough subcategory B of bijectives. Let_E_ := E/B denote its classical stable category. For example, we may take E to be the category of complexes C(A) with entries in an additive category A, in which case_E_ is the homotopy category of complexes K(A). Suppose given a finite poset D that satisfies the combinatorial condition of being ind-flat. Then, given a diagram of shape D with values in_E_ (i.e. commutative up to homotopy), there exists a diagram consisting of pure monomorphisms with values in E (i.e. commutative) that is isomorphic, as a diagram with values in_E_, to the given diagram.
Let n >= 1 and let p be a prime. Let t = 1 - zeta_{p^n}. Expand an integer j in [0,p^n-1], coprime to p, p-adically as j = sum_{s >= 0} a_s p^s. Denote the tensor product over Z_(p) by o . Then the #([0,j] - (p))th Z_(p)[t]-linear elementary divisor of the cyclotomic Dedekind embedding Z_(p)[t] o Z_(p)[t] --> prod_{i in (Z/p^n)^*} Z_(p)[t] has valuation -1 + sum_{s >= 0} (a_s (s+1) - a_{s+1} (s+2)) p^s at t. There is a similar result for the related cyclic Wedderburn embedding.
The elementary divisors of the Gram matrices of Specht modules S^lambda over the symmetric group are determined for two-row partitions and for two-column partitions lambda. More precisely, the subquotients of the Jantzen filtration are calculated using Schaper's formula. Moreover, considering a general partition lambda of n at a prime p > n - lambda_1, the only possible non trivial composition factor of S_p^lambda is induced by the morphism of Carter and Payne, as shown by means of Kleshchev's modular branching rule. This enables the Jantzen filtration to be calculated in this case as well.
We generalize certain parts of the theory of group rings to the twisted case. Let G be a finite group acting (possibly trivially) on a field L of characteristic coprime to the order of the kernel of this operation. Let K in L be the fixed field of this operation, let S be a discrete valuation ring with field of fractions K, maximal ideal generated by pi and integral closure T in L. We compute the colength of the twisted group ring T G in a maximal order in L G. Moreover, if S/pi S is finite, we compute the S/pi S- dimension of the center of T G/Jac(T G). If this quotient is split semisimple, this yields a formula for the number of simple T G-modules, generalizing Brauer's formula.
We construct set-valued right Kan-extensions via a relative Yoneda Lemma.
Let n geq 1, let lambda be a partition of n, let mu be a partition arising from lambda by a downwards shift of two boxes situated at the bottom of a column. We give a formula for a ZS_n-linear morphism of order m between the corresponding Specht modules over Z/(m), where m is the box shift length (divided by two in certain combinatorially specified cases). Reformulated, this yields an extension of the corresponding Specht modules over Z of order m in Ext^1.