In
mathematical logic and
computer science the symbol ⊢ () has taken the name turnstile because of its resemblance to a typical
turnstile if viewed from above. It is also referred to as tee and is often read as "yields", "proves", "satisfies" or "entails".
In
epistemology,
Per Martin-Löf (1996) analyzes the symbol thus: "...[T]he combination of Frege's Urteilsstrich, judgement stroke [ | ], and Inhaltsstrich, content stroke [—], came to be called the assertion sign."[1] Frege's notation for a
judgement of some content A
Consistent with its use for derivability, a "⊢" followed by an expression without anything preceding it denotes a
theorem, which is to say that the expression can be derived from the rules using an
empty set of
axioms. As such, the expression
means that Q is a theorem in the system.
In
proof theory, the turnstile is used to denote "provability" or "derivability". For example, if T is a
formal theory and S is a particular sentence in the language of the theory then
means that S is
provable from T.[4] This usage is demonstrated in the article on
propositional calculus. The syntactic consequence of provability should be contrasted to semantic consequence, denoted by the
double turnstile symbol . One says that is a semantic consequence of , or , when all possible
valuations in which is true, is also true. For propositional logic, it may be shown that semantic consequence and derivability are equivalent to one-another. That is, propositional logic is sound ( implies ) and complete ( implies )[5]
In
sequent calculus, the turnstile is used to denote a
sequent. A sequent asserts that, if all the antecedents are true, then at least one of the consequents must be true.
In the
typed lambda calculus, the turnstile is used to separate typing assumptions from the typing judgment.[6][7]
In
category theory, a reversed turnstile (), as in , is used to indicate that the
functorF is
left adjoint to the functor G.[8] More rarely, a turnstile (), as in , is used to indicate that the functor G is
right adjoint to the functor F.[9]
In
APL the symbol is called "right tack" and represents the ambivalent right identity function where both X⊢Y and ⊢Y are Y. The reversed symbol "⊣" is called "left tack" and represents the analogous left identity where X⊣Y is X and ⊣Y is Y.[10][11]
In
Hewlett-Packard's
HP-41C/
CV/
CX and
HP-42S series of calculators, the symbol (at code point 127 in the
FOCAL character set) is called "Append character" and is used to indicate that the following characters will be appended to the alpha register rather than replacing the existing contents of the register. The symbol is also supported (at code point 148) in a
modified variant of the
HP Roman-8 character set used by other HP calculators.
In
LaTeX there is a turnstile package which issues this sign in many ways, and is capable of putting labels below or above it, in the correct places.[16]
Similar graphemes
꜔ (U+A714) Modifier Letter Mid Left-Stem Tone Bar
├ (U+251C) Box Drawings Light Vertical And Right
In
mathematical logic and
computer science the symbol ⊢ () has taken the name turnstile because of its resemblance to a typical
turnstile if viewed from above. It is also referred to as tee and is often read as "yields", "proves", "satisfies" or "entails".
In
epistemology,
Per Martin-Löf (1996) analyzes the symbol thus: "...[T]he combination of Frege's Urteilsstrich, judgement stroke [ | ], and Inhaltsstrich, content stroke [—], came to be called the assertion sign."[1] Frege's notation for a
judgement of some content A
Consistent with its use for derivability, a "⊢" followed by an expression without anything preceding it denotes a
theorem, which is to say that the expression can be derived from the rules using an
empty set of
axioms. As such, the expression
means that Q is a theorem in the system.
In
proof theory, the turnstile is used to denote "provability" or "derivability". For example, if T is a
formal theory and S is a particular sentence in the language of the theory then
means that S is
provable from T.[4] This usage is demonstrated in the article on
propositional calculus. The syntactic consequence of provability should be contrasted to semantic consequence, denoted by the
double turnstile symbol . One says that is a semantic consequence of , or , when all possible
valuations in which is true, is also true. For propositional logic, it may be shown that semantic consequence and derivability are equivalent to one-another. That is, propositional logic is sound ( implies ) and complete ( implies )[5]
In
sequent calculus, the turnstile is used to denote a
sequent. A sequent asserts that, if all the antecedents are true, then at least one of the consequents must be true.
In the
typed lambda calculus, the turnstile is used to separate typing assumptions from the typing judgment.[6][7]
In
category theory, a reversed turnstile (), as in , is used to indicate that the
functorF is
left adjoint to the functor G.[8] More rarely, a turnstile (), as in , is used to indicate that the functor G is
right adjoint to the functor F.[9]
In
APL the symbol is called "right tack" and represents the ambivalent right identity function where both X⊢Y and ⊢Y are Y. The reversed symbol "⊣" is called "left tack" and represents the analogous left identity where X⊣Y is X and ⊣Y is Y.[10][11]
In
Hewlett-Packard's
HP-41C/
CV/
CX and
HP-42S series of calculators, the symbol (at code point 127 in the
FOCAL character set) is called "Append character" and is used to indicate that the following characters will be appended to the alpha register rather than replacing the existing contents of the register. The symbol is also supported (at code point 148) in a
modified variant of the
HP Roman-8 character set used by other HP calculators.
In
LaTeX there is a turnstile package which issues this sign in many ways, and is capable of putting labels below or above it, in the correct places.[16]
Similar graphemes
꜔ (U+A714) Modifier Letter Mid Left-Stem Tone Bar
├ (U+251C) Box Drawings Light Vertical And Right