2.4.15: Summary

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

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

Proof systems provide purely syntactic methods for characterizing consequence and compatibility between sentences. The sequent calculus is one such proof system. A derivation in it consists of a tree of sequents (a sequent \(\Gamma \Sequent \Delta\) consists of two sequences of formulas separated by \(\Sequent\)). The topmost sequents in a derivation are initial sequents of the form \(A \Sequent A\). All other sequents, for the derivation to be correct, must be correctly justified by one of a number of inference rules. These come in pairs; a rule for operating on the left and on the right side of a sequent for each connective and quantifier. For instance, if a sequent \(\Gamma \Sequent \Delta, A \lif B\) is justified by the \(\RightR{\lif}\) rule, the preceding sequent (the premise) must be \(A, \Gamma \Sequent \Delta, B\). Some rules also allow the order or number of sentences in a sequent to be manipulated, e.g., the \(\RightR{\Exchange}\) rule allows two formulas on the right side of a sequent to be switched.

If there is a derivation of the sequent \(\quad \Sequent A\), we say \(A\) is a theorem and write \(\Proves A\). If there is a derivation of \(\Gamma_0 \Sequent A\) where every \(B\) in \(\Gamma_0\) is in \(\Gamma\), we say \(A\) is derivable from \(\Gamma\) and write \(\Gamma \Proves A\). If there is a derivation of \(\Gamma_0 \Sequent \quad\) where every \(B\) in \(\Gamma_0\) is in \(\Gamma\), we say \(\Gamma\) is inconsistent, otherwise consistent. These notions are interrelated, e.g., \(\Gamma \Proves A\) iff \(\Gamma \cup \{\lnot A\}\) is inconsistent. They are also related to the corresponding semantic notions, e.g., if \(\Gamma \Proves A\) then \(\Gamma \Entails A\). This property of proof systems—what can be derived from \(\Gamma\) is guaranteed to be entailed by \(\Gamma\)—is called soundness. The soundness theorem is proved by induction on the length of derivations, showing that each individual inference preserves validity of the conclusion sequent provided the premise sequents are valid.