•Let s, s’ be two disjoint
vocabularies, and C a class of s’-structures (C µ STRUCT[s’])
•
•Definition A sentence f over s [ s’ is called C-invariant if
for any A 2 STRUCT[s] and A’, A’’ 2 C, s.t. A=A’=A”
(i.e. same universes)
•
(A,A’) ² f iff
(A,A’’) ² f
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•Notation f 2 (FO+C)inv