Whitehead problem
An ordinary-looking question in algebra turned out to have no answer in standard set theory. It is true under V = L and false under Martin's axiom with the continuum hypothesis false. It was the first major problem from outside logic shown independent.
Evidence tier V3 is the import default for a result the curation records as settled in the published record. No formal proof or certificate is attached to this page yet.
Posed 1950s, J. H. C. Whitehead. Closed 1974, Saharon Shelah: independent of ZFC.
Sources
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