On reduction maps and support problem in K-theory and abelian varieties

Abstract

In this paper we consider reduction maps $r_{v} : K_{2n+1}(F)/C_{F} \to K_{2n+1}(κ_{v})_{l}$ where $F$ is a number field and $C_{F}$ denotes the subgroup of $K_{2n+1}(F)$ generated by $l$-parts (for all primes $l$) of kernels of the Dwyer-Friedlander map and maps $r_{v} : A(F)\to A_{v}(κ_{v})_{l}$ where $A(F)$ is an abelian variety over a number field. We prove a generalization of the support problem of Schinzel for $K$-groups of number fields: Let $P_{1}, ..., P_{s}, Q_{1}, ..., Q_{s}\in K_{2n+1}(F)/C_{F}$ be the points of infinite order. Assume that for almost every prime $l$ the following condition holds: for every set of positive integers $m_{1}, ..., m_{s}$ and for almost every prime $v$ $$m_{1} r_{v}(P_{1})+... + m_{s} r_{v}(P_{s})=0 \mathrm{implies} m_{1} r_{v}(Q_{1})+... + m_{s}r_{v}(Q_{s})= 0. $$ Then there exist $α_{i}$, $β_{i}\in \mathbb{Z} \setminus \{0 \}$ such that $α_{i} P_{i}+β_{i} Q_{i}=0$ in $B(F)$ for every $i \in \{1, ... s\}$. We also get an analogues result for abelian varieties over number fields. The main technical result of the paper says that if $P_{1}, ..., P_{s}$ are nontorsion elements of $K_{2n+1}(F)/C_{F}$, which are linearly independent over $\mathbb{Z}$, then for any prime $l$, and for any set $\{k_{1},... ,k_{s}\}\subset \mathbb{N} \cup \{0\}$, there are infinitely many primes $v$, such that the image of the point $P_{t}$ via the map $r_{v}$ has order equal $l^{k_{t}}$ for every $t \in \{1, ..., s \}$.