Universal classes for algebraic groups

We exhibit cocycles representing certain classes in the rational cohomology of of the general linear group with coefficients in the divided powers of a Frobenius twist of the adjoint representation. These classes' existence was anticipated by van der Kallen, and they intervene in the proof that reductive linear algebraic groups have finitely generated cohomology algebras.Comment: 24 pages. v2, slight modifications : corollary 0.2 added, additional comments in section 2, one reference added. To appear in Duke Math.

Ringel duality and derivatives of non-additive functors

We prove that Ringel duality in the category of strict polynomial functors can be interpreted as derived functors of non-additive functors (in the sense of Dold and Puppe). We give applications of this fact for both theories.Comment: Fourth version, 48 pages. Minor changes (typos corrected, comments and references added). The article is self-contained (no prior knowledge of Schur algebras, strict polynomial functors or derived functors of non-additive functors is required

Troesch complexes and extensions of strict polynomial functors

We develop a new approach of extension calculus in the category of strict polynomial functors, based on Troesch complexes. We obtain new short elementary proofs of numerous classical Ext-computations as well as new results. In particular, we get a cohomological version of the `fundamental theorems' from classical invariant invariant theory for GL_n for n big enough (and we give a conjecture for smaller values of n). We also study the `twisting spectral sequence' E^{s,t}(F,G,r) converging to the extension groups Ext^*(F^{(r)}, G^{(r)}) between the twisted functors F^{(r)} and G^{(r)}. Many classical Ext-computations simply amount to the collapsing of this spectral sequence at the second page (for lacunary reasons), and it is also a convenient tool to study the effect of the Frobenius twist on Ext-groups. We prove many cases of collapsing, and we conjecture collapsing is a general fact.Comment: Revised version, 46 pages. Mathematics unchanged. Typos corrected, Appendix 9 on Troesch complexes improve

M-curves of degree 9 with deep nests

The first part of Hilbert's sixteenth problem deals with the classification of the isotopy types realizable by real plane algebraic curves of given degree $m$. For $m \geq 8$, one restricts the study to the case of the $M$-curves. For $m=9$, the classification is still wide open. We say that an $M$-curve of degree 9 has a deep nest if it has a nest of depth 3. In the present paper, we prohibit 10 isotopy types with deep nests and no outer ovals.Comment: 16 pages, 11 figures v.4 minimal correction