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13.26 (1995)

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Is it true that a countable topological group of exponent 2 with unique free ultrafilter converging to the identity has a basis of neighborhoods of the identity consisting of subgroups?

Contributor: E. G. Zelenyuk, I. V. Protasov

Progress

Assuming Martin’s Axiom, the answer is "No" (Ye. Zelenyuk, Adv. Math., 229, no. 4 (2012), 2415–2426).

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