FOM: Conservative extensions

Joseph Shoenfield jrs at
Mon Oct 5 10:50:52 EDT 1998

     Steve's proof that ZF plus real choice is conservative over ZF for
second order arithmetic seems to be marred by a confusion between two
models of ZF.   The first, the class of sets constructible from a
real, does not necessarily satisfy choice; in fact, a popular axiom
is that this model satisfies AD.   The second, the class of sets
constructible from R (the set of reals) does not necessarily contain
all the reals.

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