Differential characters and characteristic polynomial of Frobenius.
Let an elliptic fibration with general fibre . Let be the minima of the non-zero intersection numbers where runs successively through the following sets: effective divisors on , invertible sheaves spanned by global sections, ample divisors and very ample divisors. Let be the maximum of the multiplicities of the fibres of . We prove that if and only if and that if and only if .
It is proved that any isolated singularity of complete intersection has an algebraisation whose divisor class group is finitely generated.
We prove that some of the basic differential functions appearing in the (unramified) theory of arithmetic differential equations, especially some of the basic differential modular forms in that theory, arise from a "ramified situation". This property can be viewed as a special kind of overconvergence property. One can also go in the opposite direction by using differential functions that arise in a ramified situation to construct "new" (unramified) differential functions.
We survey recent work on arithmetic analogues of ordinary and partial differential equations.
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