Computability and Logic
Computability and Logic
Computability and Logic
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25.3. NONSTANDARD MODELS OF ANALYSIS 313<br />
nonlogical symbol of L* the same denotation as does the given interpretation. Thus<br />
the lower part of N ** is N *. A sentence of L* will be true in an interpretation of L**<br />
if <strong>and</strong> only if it is true in the lower part of that interpretation. Thus a sentence of L*is<br />
a theorem of (that is, is in) true arithmetic if <strong>and</strong> only if it is a theorem of true analysis.<br />
Our first aim in this section will be to establish the existence of nonst<strong>and</strong>ard models<br />
of analysis of two distinct kinds. An interpretation of L** is called an ∈-model if (as<br />
in the st<strong>and</strong>ard interpretation) the elements of the upper domain are sets of elements<br />
of the lower domain, <strong>and</strong> the interpretation of ∈ is the membership or elementhood<br />
relation ∈ (between elements of the lower <strong>and</strong> the upper domain). The sentence<br />
∀X∀Y (∀x(x∈X ↔ x∈Y ) → X = Y )<br />
is called the axiom of extensionality. Clearly it is true in any ∈-model <strong>and</strong> hence in<br />
any model isomorphic to an ∈-model. Conversely, any model M of extensionality<br />
is isomorphic to an ∈-model M # . [To obtain M # from M, keep the same lower<br />
domain <strong>and</strong> the same interpretations for symbols of L*, replace each element α of<br />
the upper domain of M by the set α # of all elements a of the lower domain such that<br />
a ∈ M α, <strong>and</strong> interpret ∈ not as the relation ∈ M but as ∈. The identity function on the<br />
lower domain together with the function sending α to α # is an isomorphism. The only<br />
point that may not be immediately obvious is that the latter function is one-to-one. To<br />
see this, note that if α # = β # , then α <strong>and</strong> β satisfy ∀x(x ∈ X ↔ x ∈ Y )inM, <strong>and</strong><br />
since (2) is true in M, α <strong>and</strong> β must satisfy X = Y , that is, we must have α = β.]<br />
Since we are going to be interested only in models of extensionality, we may restrict<br />
our attention to ∈-models.<br />
If the lower part of an ∈-model M is the st<strong>and</strong>ard model of arithmetic, we call<br />
M an ω-model. The st<strong>and</strong>ard model of analysis is, of course, an ω-model. If an<br />
ω-model of analysis is nonst<strong>and</strong>ard, its upper domain must consist of some class<br />
of sets properly contained in the class of all sets of numbers. If the lower part of an<br />
∈-model M is isomorphic to the st<strong>and</strong>ard interpretation N *ofL*, then M as a whole<br />
is isomorphic to an ω-model M # . [If j is the isomorphism from N * to the lower part<br />
of M, replace each element α of the upper domain of M by the set of n such that<br />
j(n) ∈ α, to obtain M # .] So we may restrict our attention to models that are of one of<br />
two kinds, namely, those that either are ω-models, or have a nonst<strong>and</strong>ard lower part.<br />
Our first result is that nonst<strong>and</strong>ard models of analysis of both kinds exist.<br />
25.9 Proposition. Both nonst<strong>and</strong>ard models of analysis whose lower part is a nonst<strong>and</strong>ard<br />
model of arithmetic <strong>and</strong> nonst<strong>and</strong>ard ω-models of analysis exist.<br />
Proof: The existence of nonst<strong>and</strong>ard models of arithmetic was established in the<br />
problems at the end of Chapter 12 by applying the compactness theorem to the theory<br />
that results upon adding to arithmetic a constant ∞ <strong>and</strong> the sentences ∞ ≠ n for all<br />
natural numbers n. The same proof, with analysis in place of arithmetic, establishes<br />
the existence of a nonst<strong>and</strong>ard model of analysis whose lower parts is a nonst<strong>and</strong>ard<br />
model of arithmetic. The strong Löwenheim–Skolem theorem implies the existence<br />
of an enumerable subinterpretation of the st<strong>and</strong>ard model of analysis that is itself<br />
a model of analysis. This must be an ω-model, but it cannot be isomorphic to the<br />
st<strong>and</strong>ard model, whose upper domain is nonenumerable.