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Frobenius direct images of line bundles on toric varieties. (English) Zbl 0957.14036

Let \(X\) be a scheme over an algebraically closed field of characteristic \(p>0\). Recall that the absolute Frobenius morphism \(F:X\to X\) is the morphism which is the identity on the underlying point set of \(X\) and the \(p\)-th power map at the sheaf level. The author proves that if \(X\) is a smooth toric variety and \(L\) is a line bundle on \(X\), then the direct image \(F_*L\) of \(L\) via \(F_*\) splits into a direct sum of line bundles. This generalizes on old result of Hartshorne for \(X=\mathbb{P}^n\). Thomsen’s proof is constructive and gives an algorithm to compute the decomposition explicitly. R. Bøgvad [Proc. Am. Math. Soc. 126, No. 12, 3447-3454 (1998; Zbl 0902.14038)] has further generalized this result of Thomsen by showing that for a \(T\)-linearized vector bundle \(M\) in the toric variety \(X\) (under \(T)\), \(F_*M\) splits into a sum of vector bundles of the same rank as \(M\).

MSC:

14M25 Toric varieties, Newton polyhedra, Okounkov bodies
14G25 Global ground fields in algebraic geometry
14C20 Divisors, linear systems, invertible sheaves
14F05 Sheaves, derived categories of sheaves, etc. (MSC2010)

Citations:

Zbl 0902.14038
Full Text: DOI

References:

[1] Bøgvad, R., Splitting of the direct image of sheaves under the Frobenius, Proc. Amer. Math. Soc., 126, 3447-3454 (1998) · Zbl 0902.14038
[2] Fulton, W., Introduction to Toric Varieties (1993), Princeton Univ. Press: Princeton Univ. Press Princeton · Zbl 0813.14039
[3] Hartshorne, R., Ample Subvarieties of Algebraic Varieties (1970), Springer-Verlag: Springer-Verlag Berlin/New York · Zbl 0208.48901
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