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Published online by Cambridge University Press: 12 March 2014
It is well known that every Boolean algebra of size ω1 can be embedded into (ω)/fin. E. van Douwen proved that if CH failed then there is a Boolean algebra of size ω2 which cannot be embedded into (ω1)/ctble. We show that ◇ is equivalent to the statement that a certain natural Boolean algebra of size embeds into (ω1)/ctble. I would like to thank B. Velickovic for many helpful conversations.