| 1. |
- Bridy, Andrew, et al.
(författare)
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Finite ramification for preimage fields of post-critically finite morphisms
- 2017
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Ingår i: Mathematical Research Letters. - : International Press of Boston, Inc.. - 1073-2780 .- 1945-001X. ; 24:6, s. 1633-1647
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Tidskriftsartikel (refereegranskat)abstract
- Given a finite endomorphism phi of a variety X defined over the field of fractions K of a Dedekind domain, we study the extension K (phi(-infinity)(alpha)) := boolean OR(n >= 1) K (phi(-n) (alpha)) generated by the preimages of alpha under all iterates of phi. In particular when phi is post-critically finite, i.e., there exists a non-empty, Zariski-open W subset of X such that phi(-1) (W) subset of W and phi : W -> X is etale, we prove that K (phi(-infinity) (alpha)) is rami fied over only finitely many primes of K. This provides a large supply of in finite extensions with restricted rami fication, and generalizes results of Aitken-Hajir-Maire [1] in the case X = A(1) and Cullinan-Hajir, Jones-Manes [7, 13] in the case X = P-1. Moreover, we conjecture that this finite rami fication condition characterizes post-critically finite morphisms, and we give an entirely new result showing this for X = P-1. The proof relies on Faltings' theorem and a local argument.
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| 2. |
- Jones, Rafe, et al.
(författare)
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Eventually stable rational functions
- 2017
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Ingår i: International Journal of Number Theory. - : WORLD SCIENTIFIC PUBL CO PTE LTD. - 1793-0421. ; 13:9, s. 2299-2318
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Tidskriftsartikel (refereegranskat)abstract
- For a field K, rational function phi is an element of K(z) of degree at least two, and alpha is an element of P-1(K), we study the polynomials in K[z] whose roots are given by the solutions in K to phi(n)(z) = a, where fn denotes the nth iterate of phi. When the number of irreducible factors of these polynomials stabilizes as n grows, the pair (phi, alpha) is called eventually stable over K. We conjecture that (phi, alpha) is eventually stable over K when K is any global field and a is any point not periodic under f (an additional non-isotriviality hypothesis is necessary in the function field case). We prove the conjecture when K has a discrete valuation for which (1) f has good reduction and (2) facts bijectively on all finite residue extensions. As a corollary, we prove for these maps a conjecture of Sookdeo on the finiteness of S-integral points in backward orbits. We also give several characterizations of eventual stability in terms of natural finiteness conditions, and survey previous work on the phenomenon.
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