2.2.6: Expressing the Size of Structures

\(\def\Assign#1#2{ { #1^{\Struct{#2}} } }\)
\(\def\Atom#1#2{ { \mathord{#1}(#2) } }\)
\(\def\Bin{ {\mathbb{B}} }\)
\(\def\cardeq#1#2{ { #1 \approx #2 } }\)
\(\def\cardle#1#2{ { #1 \preceq #2 } }\)
\(\def\cardless#1#2{ { #1 \prec #2 } }\)
\(\def\cardneq#1#2{ { #1 \not\approx #2 } }\)
\(\def\comp#1#2{ { #2 \circ #1 } }\)
\(\def\concat{ { \;\frown\; } }\)
\(\def\Cut{ { \text{Cut} } }\)
\(\def\Discharge#1#2{ { [#1]^#2 } }\)
\(\def\DischargeRule#1#2{ { \RightLabel{#1}\LeftLabel{\scriptsize{#2} } } }\)
\(\def\dom#1{ {\operatorname{dom}(#1)} }\)
\(\def\Domain#1{ {\left| \Struct{#1} \right|} }\)
\(\def\Elim#1{ { {#1}\mathrm{Elim} } }\)
\(\newcommand{\Entails}{\vDash}\)
\(\newcommand{\EntailsN}{\nvDash}\)
\(\def\eq[#1][#2]{ { #1 = #2 } }\)
\(\def\eqN[#1][#2]{ { #1 \neq #2 } }\)
\(\def\equivclass#1#2{ { #1/_{#2} } }\)
\(\def\equivrep#1#2{ { [#1]_{#2} } }\)
\(\def\Exchange{ { \text{X} } }\)
\(\def\False{ { \mathbb{F} } }\)
\(\def\FalseCl{ { \lfalse_C } }\)
\(\def\FalseInt{ { \lfalse_I } }\)
\(\def\fCenter{ { \,\Sequent\, } }\)
\(\def\fdefined{ { \;\downarrow } }\)
\(\def\fn#1{ { \operatorname{#1} } }\)
\(\def\Frm[#1]{ {\operatorname{Frm}(\Lang #1)} }\)
\(\def\fundefined{ { \;\uparrow } }\)
\(\def\funimage#1#2{ { #1[#2] } }\)
\(\def\funrestrictionto#1#2{ { #1 \restriction_{#2} } }\)
\(\newcommand{\ident}{\equiv}\)
\(\newcommand{\indcase}[2]{#1 \ident #2\text{:}}\)
\(\newcommand{\indcaseA}[2]{#1 \text{ is atomic:}}\)
\(\def\indfrm{ { A } }\)
\(\def\indfrmp{ { A } }\)
\(\def\joinrel{\mathrel{\mkern-3mu}}\)
\(\def\lambd[#1][#2]{\lambda #1 . #2}\)
\(\def\Lang#1{ { \mathcal{#1} } }\)
\(\def\LeftR#1{ { {#1}\mathrm{L} } }\)
\(\def\len#1{ {\operatorname{len}(#1)} }\)
\(\def\lexists#1#2{ { \exists #1\, #2 } }\)
\(\def\lfalse{ {\bot} }\)
\(\def\lforall#1#2{ { \forall#1\, #2 } }\)
\(\newcommand{\lif}{\rightarrow}\)
\(\newcommand{\liff}{\leftrightarrow}\)
\(\def\Log#1{ { \mathbf{#1} } }\)
\(\def\ltrue{ {\top} }\)
\(\def\Id#1{ {\operatorname{Id}_#1} }\)
\(\def\Int{ {\mathbb{Z}} }\)
\(\def\Intro#1{ { {#1}\mathrm{Intro} } }\)
\(\def\mModel#1{ { \mathfrak{#1} } }\)
\(\newcommand{\mSat}[3][{}]{\mModel{#2}{#1}\Vdash{#3}}\)
\(\newcommand{\mSatN}[3][{}]{\mModel{#2}{#1}\nVdash{#3}}\)
\(\def\Nat{ {\mathbb{N}} }\)
\(\def\nicefrac#1#2{ {{}^#1/_#2} }\)
\(\def\num#1{ { \overline{#1} } }\)

\(\def\ran#1{ {\operatorname{ran}(#1)} }\)
\(\newcommand{\Obj}[1]{\mathsf{#1}}\)
\(\def\Rat{ {\mathbb{Q}} }\)
\(\def\Real{ {\mathbb{R}} }\)
\(\def\RightR#1{ { {#1}\mathrm{R} } }\)
\(\def\Part#1#2{ { \Atom{\Obj P}{#1, #2} } }\)
\(\def\pto{ { \hspace{0.1 cm}\to\hspace{-0.44 cm}\vcenter{\tiny{\hbox{|}}}\hspace{0.35 cm} } }\)
\(\def\PosInt{ {\mathbb{Z}^+} }\)
\(\def\Pow#1{ {\wp(#1)} }\)
\(\newcommand{\Proves}{\vdash}\)
\(\newcommand{\ProvesN}{\nvdash}\)
\(\def\Relbar{\mathrel{=}}\)
\(\newcommand{\Sat}[3][{}]{\Struct{#2}{#1}\vDash{#3}}\)
\(\newcommand{\SatN}[3][{}]{\Struct{#2}{#1}\nvDash{#3}}\)
\(\newcommand{\Sequent}{\Rightarrow}\)
\(\def\Setabs#1#2{ { \{#1:#2\} } }\)
\(\newcommand{\sFmla}[2]{#1\,#2}\)
\(\def\Struct#1{ {#1} }\)
\(\def\subst#1#2{ { #1/#2 } }\)
\(\def\Subst#1#2#3{ { #1[\subst{#2}{#3}] } }\)
\(\def\TMblank{ { 0 } }\)
\(\newcommand{\TMendtape}{\triangleright}\)
\(\def\TMleft{ { L } }\)
\(\def\TMright{ { R } }\)
\(\def\TMstay{ { N } }\)
\(\def\TMstroke{ { 1 } }\)
\(\def\TMtrans#1#2#3{ { #1,#2,#3 } }\)
\(\def\Trm[#1]{ {\operatorname{Trm}(\Lang #1)} }\)
\(\def\True{ { \mathbb{T} } }\)
\(\newcommand{\TRule}[2]{#2#1}\)
\(\def\tuple#1{ {\langle #1 \rangle} }\)
\(\newcommand{\Value}[3][\,]{\mathrm{Val}_{#1}^{#3}(#2)}\)
\(\def\Var{ { \mathrm{Var} } }\)
\(\newcommand{\varAssign}[3]{#1 \sim_{#3} #2}\)
\(\def\Weakening{ { \text{W} } }\)

There are some properties of structures we can express even without using the non-logical symbols of a language. For instance, there are sentences which are true in a structure iff the domain of the structure has at least, at most, or exactly a certain number \(n\) of elements.

Proposition \(\PageIndex{1}\)

The sentence \[\begin{gathered} A_{\ge n} \ident \lexists{x_1}{\lexists{x_2}{\dots\lexists{x_n}{}}}\hspace{288px}\\ \hspace{108px}\begin{aligned} (\eqN[x_1][x_2] \land {} \eqN[x_1][x_3] \land \eqN[x_1][x_4] \land \dots \land \eqN[x_1][x_n] \land {}\\ \eqN[x_2][x_3] \land \eqN[x_2][x_4] \land \dots \land {} \eqN[x_2][x_n] \land {} \\ \vdots\\ \eqN[x_{n-1}][x_n]) \end{aligned}\end{gathered}\] is true in a structure \(\Struct M\) iff \(\Domain M\) contains at least \(n\) elements. Consequently, \(\Sat{M}{\lnot A_{\ge n+1}}\) iff \(\Domain M\) contains at most \(n\) elements.

Proposition \(\PageIndex{2}\)

The sentence \[\begin{gathered} A_{= n} \ident \lexists{x_1}{\lexists{x_2}{\dots\lexists{x_n}{}}}\hspace{288px} \\ \hspace{108px}\begin{aligned} (\eqN[x_1][x_2] \land {} \eqN[x_1][x_3] \land \eqN[x_1][x_4] \land \dots \land \eqN[x_1][x_n] \land {}\\ \eqN[x_2][x_3] \land \eqN[x_2][x_4] \land \dots \land {} \eqN[x_2][x_n] \land {} \\ \vdots\\ \eqN[x_{n-1}][x_n] \land {} \\ \lforall{y}{(\eq[y][x_1] \lor \dots \lor \eq[y][x_n]})) \end{aligned}\end{gathered}\] is true in a structure \(\Struct M\) iff \(\Domain M\) contains exactly \(n\) elements.

Proposition \(\PageIndex{3}\)

A structure is infinite iff it is a model of \[\{A_{\ge 1}, A_{\ge 2}, A_{\ge 3}, \dots \}.\nonumber\]

There is no single purely logical sentence which is true in \(\Struct M\) iff \(\Domain M\) is infinite. However, one can give sentences with non-logical predicate symbols which only have infinite models (although not every infinite structure is a model of them). The property of being a finite structure, and the property of being a non-enumerable structure cannot even be expressed with an infinite set of sentences. These facts follow from the compactness and Löwenheim-Skolem theorems.