Singurality Theory and Frobenius Morphism

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Frobenius endomorphism hi characteristic p > 0 is a very powerful tool in commutative ring theory as well as singularity theory or algebraic geometry iover a field of characteristic 0 via reduction mod p.We applied the Frobenius endomorphism to various problems in commutative algebra and singularity theory. Inparticular, we showed the followings ;1. F-thresholds ; F-threshold is defined in a Noetherian ring of characteristic p>0 to a pair (I,J) of two ideals of A.This notion was originally introduced to describe multiplier ideals and jumping numbers in a regular local ring. But in our research, it turned out that this notion is closely related to tight closures and integral closures and also we have a nice conjecture concerning F-threshold and multiplicity of a parameter ideal.2. Multi-graded rings, rational singularity and F-rational rings ;The notion of multi-graded rings and their diagonal algebras is a very interesting object and very useful in making many interesting examples. In a joint paper with A Singh and E. Sato, K. Kurano and K. Watanabe made a new example with discrete divisor class group whose local cohomology modules shows very interesting feature. Also we showed a criterion for diagonal subalgebras of multi-graded hypersurfaces to be f-rational or F-regular in terms of the degree.3. Totally reflexive modules ;In the theory of totally reflexive modules, examples of non-trivial totally reflexive modules are very few. Watanabe and R. Takahashi constructed a family of non-trivial totally reflexive modules using geometry of curves of genus greater than 1. This is the first case that algebraic geometry is used in this theory.