Let $X$ be a variety over an algebraically closed field $k$ of positive characteristic $p$. Let $F : X \to X$ denote the absolute Frobenius morphism, i.e. the morphism that is the identity on points and whose comorphism is the $p^{th}$ power map. We say that $X$ is Frobenius split if the $p^{th}$ power morphism $ \mathcal O_X \to F_* \mathcal O_X $ splits (in the category of sheaves of $\mathcal O_X$-modules).

Let $G$ be a semisimple algebraic group over $k$ and let $B \subseteq G$ be a Borel subgroup. A number of varieties associated to $G$ are known to be Frobenius split: the flag variety $G/B$, the cotangent bundle $T^*(G/B)$, etc.


Is it known in general whether or not the tangent bundle $T(G/B)$ to $G/B$ is Frobenius split? I think that the splitting of $T(G/B)$ would be implied by a splitting of $X \times X$ that maximally compatibly splits the diagonal, so by recent work of Lauritzen and Thomsen (in fact, the proof in type $C$ was just posted on the arXiv), I believe one does know that $T(G/B)$ is Frobenius split in types $A$ and $C$. Does anyone know of any other work exploring this topic?

  • $\begingroup$ Success in types A and C is encouraging for the general case, but I don't know how to get there. If the Aarhus gang don't yet know how to, probably nobody else does. But your question seems reasonable. $\endgroup$ – Jim Humphreys Sep 3 '10 at 19:07
  • $\begingroup$ According to the Brion-Kumar book, it is known that the cotangent bundle to $G/B$ is Frobenius split. $\endgroup$ – Victor Protsak Sep 4 '10 at 5:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.