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12.44 (1992)

Solved

(P. Hall). Is there a non-trivial group which is isomorphic with every proper extension of itself by itself?

Contributor: J. C. Lennox

Progress

Yes, such groups exist of cardinality any regular cardinal (R. Göbel, S. Shelah, Math. Proc. Cambridge Philos. Society (2), 134, no. 1 (2003), 23–31).

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