Kneser’s theorem in -finite abelian groups

نویسندگان

چکیده

Abstract Let G be a $\sigma $ -finite abelian group, i.e., $G=\bigcup _{n\geq 1} G_n$ where $(G_n)_{n\geq 1}$ is nondecreasing sequence of finite subgroups. For any $A\subset G$ , let $\underline {\mathrm {d}}( A ):=\liminf _{n\to \infty }\frac {|A\cap G_n|}{|G_n|}$ its lower asymptotic density. We show that for subsets and B whenever A+B )<\underline )+\underline )$ the sumset $A+B$ must periodic, is, union translates subgroup $H\leq index. This exactly analogous to Kneser’s theorem regarding density infinite sets integers. Further, we similar statements upper in case $A=\pm B$ . An analagous statement had already been proven by Griesmer very general context countable groups, but present paper provides much simpler argument specifically tailored setting groups. relies on an appeal another Kneser, namely one sumsets group.

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ژورنال

عنوان ژورنال: Canadian mathematical bulletin

سال: 2022

ISSN: ['1496-4287', '0008-4395']

DOI: https://doi.org/10.4153/s0008439521001053