English

Good reduction and Shafarevich-type theorems for dynamical systems with portrait level structures

Number Theory 2018-03-28 v3 Algebraic Geometry Dynamical Systems

Abstract

Let KK be a number field, let SS be a finite set of places of KK, and let RSR_S be the ring of SS-integers of KK. A KK-morphism f:PK1PK1f:\mathbb{P}^1_K\to\mathbb{P}^1_K has simple good reduction outside SS if it extends to an RSR_S-morphism PRS1PRS1\mathbb{P}^1_{R_S}\to\mathbb{P}^1_{R_S}. A finite Galois invariant subset XPK1(Kˉ)X\subset\mathbb{P}^1_K(\bar{K}) has good reduction outside SS if its closure in PRS1\mathbb{P}^1_{R_S} is \'etale over RSR_S. We study triples (f,Y,X)(f,Y,X) with X=Yf(Y)X=Y\cup f(Y). We prove that for a fixed KK, SS, and dd, there are only finitely many PGL2(RS)\text{PGL}_2(R_S)-equivalence classes of triples with deg(f)=d\text{deg}(f)=d and PYef(P)2d+1\sum_{P\in Y}e_f(P)\ge2d+1 and XX having good reduction outside SS. We consider refined questions in which the weighted directed graph structure on f:YXf:Y\to X is specified, and we give an exhaustive analysis for degree 22 maps on P1\mathbb{P}^1 when Y=XY=X.

Keywords

Cite

@article{arxiv.1703.00823,
  title  = {Good reduction and Shafarevich-type theorems for dynamical systems with portrait level structures},
  author = {Joseph H. Silverman},
  journal= {arXiv preprint arXiv:1703.00823},
  year   = {2018}
}

Comments

47 pages - the original version of this paper is being split into two pieces. This piece contains the material on dynamical Shafarevich theorems. The second part, which will be posted separately, will contain material on moduli spaces for dynamical systems with portraits

R2 v1 2026-06-22T18:33:45.168Z