[FOM] First order axiomatizability

pax0 at seznam.cz pax0 at seznam.cz
Thu Nov 24 14:40:09 EST 2011


Does someone know where there is a proof (and whose result it is) the following:
A class of first order strucutres is fisrt order axiomatizable 
iff
it is closed under isomorphisms and ultrapowers.
Thank you Jan Pax


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