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arxiv: 1609.08900 · v2 · pith:4CFPXG2Jnew · submitted 2016-09-28 · 🧮 math.GR

Gradients of sequences of subgroups in a direct product

classification 🧮 math.GR
keywords inftydirectfinitefinitelyfracgroupsmathrmproduct
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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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