# All Finite S-structures satisfying “characterizing” S-sentence are isomorphic

Excercise 5.9, p.44 Ebbinghaus et al - Mathematical logic:

This excercise was discussed here: Can a single sentence be used to distinguish between isomorphic classes of finite structures?

Roughly speaking, we can find a sentence phi which incorporates all rules given by structure A (existence and uniqueness of all elements of the underlying domain, by A defined functions, by A defined constants, by A defined relations), so that A and an isomorphic structures B satisfies phi.

It is asserted that the converse is true, either, i.e. the structures satisfying phi are precisely those isomorphic to A. But what if I take A and extend A by another function/constant/relation and thereby obtain a structure B.

The sentence phi is still satisfied by B but A and B are not isomorphic. I guess I am missing/forgetting some detail of a definition or so.

Your title contains the key point:

All Finite S-structures satisfying “characterizing” S-sentence are isomorphic

(emphasis mine).

what if I take A and extend A by another function/constant/relation and thereby obtain a structure B

is, such a $$B$$ is not an $$S$$-structure, and so the result quoted doesn't apply.

A version of the quoted result which holds for all language would be:

For every finite language $$S$$ and every finite $$S$$-structure $$A$$, there is an $$S$$-sentence $$\varphi$$ such that every structure $$B$$ which satisfies $$\varphi$$ has $$B\upharpoonright S\cong A$$ - that is, the reduct of $$B$$ to the original language $$S$$ is isomorphic to $$A$$ (even if $$B$$ itself isn't, by virtue of being a structure in a too-big language).

• Thanks, I made the exact mistake, not affiliating S-structure to that specific S a few excercises ago, leaving myself with hours of confusion. Anyway, a few pages earlier it is stated, that S-structures, which satisfy the same S-sentences are in general not isomorphic. I infer correctly, that that could only be the case when discussing infinite S-structuers. In your answer you use the term "finite language S". What do you mean by that ? If I consider a finite S-structure, the language can still be countable, depending on the underlying syntactic calculus. – Quantaurix Feb 28 '19 at 7:00
• @Quantaurix If the language is infinite, the statement isn't true, and this is a good exercise. (HINT - note that a single sentence can only mention finitely many symbols ...) – Noah Schweber Feb 28 '19 at 22:58
• Thanks, I got it. As long as there are finitely many elements in $S$ and finitely many elements in the domain of $S$-structure $A$, one can obtain a formula $\varphi$ which contains all information. If the language is infinite, i.e. there are at least countable many elements in the domain of $A$, a finite formula $\varphi$ cannot exist by the reason mentioned in your comment. – Quantaurix Mar 4 '19 at 12:24

When we say a sentence $$\phi$$ axiomatises a class of structures, we always have a particular signature in mind. It doesn't make sense to ask whether structures with different signatures are isomorphic, e.g., a ring can't be isomorphic to a group, because the ring has operations that the group doesn't have. A sentence like $$\forall x \forall y. xy = yx$$ does indeed characterize both commutative groups and commutative rings, but we don't use it to compare a group with a ring.