# Gradients of sequences of subgroups in a direct product

Nikolov, N
Shemtov, Z
Shusterman, M

9 October 2017

## Journal:

International Mathematics Research Notices

## Last Updated:

2020-06-08T11:45:37.61+01:00

## DOI:

10.1093/imrn/rnx236

## abstract:

For a sequence $\{U_n\}_{n = 1}^\infty$ of finite index subgroups of a direct
product $G = A \times B$ of finitely generated groups, we show that $$\lim_{n \to \infty} \frac{\min\{|X| : \langle X \rangle = U_n\}}{[G : U_n]} = 0$$ once
$[A : A \cap U_n], [B : B \cap U_n] \to \infty$ as $n \to \infty$. Our proof
relies on the classification of finite simple groups. For $A,B$ that are
finitely presented we show that $$\lim_{n \to \infty} \frac{\log |\mathrm{Torsion}(U_n^{\mathrm{ab}})|}{[G : U_n]} = 0.$$

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