• 한국수학교육학회지시리즈B:순수및응용수학
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• 제13권4호
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• pp.269-280
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• 2006

Recently $L_{\delta}$-groups were introduced in the study of geometric group theory. Three levels of $L_{\delta}$-groups are difined and discussed. It is shown that each of these levels of $L_{\delta}$-groups is closed under taking a direct product.