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The following is simply an endeavour to explore alternative ways of writing quantifier notation for fragments of first\higher order logic.

  1. The arrow notation:

$\uparrow \! x, \phi \iff \forall x \, \phi \\ \downarrow \! x , \phi \iff \exists x \, \phi\\ {\overset {\large \downarrow} .} \, x, \phi \iff \exists ! x \, \phi$

Examples:

Extensionality: $\uparrow \! x , \uparrow \! y, (\uparrow \! z , (z \in x \leftrightarrow z \in y) \to x=y)$

Foundation: $\downarrow \! x , (x \in y ) \to , \downarrow \! x, (x \in y \land \neg \downarrow \! z , (z \in x \land z \in y))$

Union: $\uparrow \! A, \downarrow \! x, \uparrow \! y , (y \in x \leftrightarrow , \downarrow \! z , (z \in A \land y \in z))$

Power: $\uparrow \! A, \downarrow \! x, \uparrow \! y , (y \in x \leftrightarrow , \uparrow \! z , (z \in y \to y \in A))$

Separation: $\uparrow \! A, \downarrow \! x, \uparrow \! y , (y \in x \leftrightarrow y \in A \land \phi(y,A))$

Replacement: $\uparrow \! A, \ (\uparrow \! x \in A, {\overset {\large \downarrow} .} \, y, \phi(x,y) \to \\\downarrow \! B, \uparrow \!y , (y \in B \leftrightarrow \, \, \downarrow \! x, (x \in A \land \phi(x,y))))$

Infinity: $\downarrow \! x, : \varnothing \in x \ \land \uparrow \! y , (y \in x \to \{y\} \in x)$

Choice: $\uparrow \! X, \ [ \uparrow \!m, \uparrow \! n , (m \in X \land n \in X\to \, \, \neg \downarrow \! l, : l \in m \land l \in n) \\\to, \downarrow \! C, \uparrow \! x \in X, {\overset {\large \downarrow} .} \, c, : c \in C \land c \in x] $

I'm still of the opinion that this way of writing quantifiers is the simplest appropriate way of doing it. The pitfull is that it doesn't easily illustrate a spectrum of quantification concepts as the following alternative does:

  1. The circle notation:

${\LARGE \bullet} x \, \phi \iff \forall x \, \phi$

$^{\large \odot} x \, \phi \iff \neg \, {\LARGE \bullet } x \, \neg \phi$

Examples:

Choice: ${\LARGE \bullet} \! X \ [ {\LARGE \bullet} m, {\LARGE \bullet} n \, (m \in X \land n \in X\to \, \, \neg \, ^{\large \odot} l: l \in m \land l \in n) \to \\ ^{\large \odot}\! C, {\LARGE \bullet} x \in X, ^{\large \odot !} \! c: c \in C \land c \in x] $

Infinity: $^{\large \odot}\! x: \varnothing \in x \ \land {\LARGE \bullet} y \in x \, (\{y\} \in x)$

Replacement: ${\LARGE \bullet} A \ ({\LARGE \bullet} x \in A, ^{\large \odot !} \! y: \phi(x,y) \to \\^{\large \odot} \! B, {\LARGE \bullet} y \, (y \in B \leftrightarrow \, ^{\large \odot} x \in A: \phi(x,y)))$

This notation have the blessing of showing a spectrum of quantification, as follows:

$^\bigcirc x \, \phi \iff {\LARGE \bullet} x \, \neg \phi$; read as: Non x

$^{\Huge \circ}\kern-0.930 em ^{\huge \bullet} \! x \, \phi \iff |\{x \mid \phi \}| > |\{x \mid \neg \phi\}|$; read as: Most $x$

$^{\Huge \circ}\kern-0.780em ^{\Large \bullet}\! x \, \phi \iff |\{x \mid \phi \}| < |\{x \mid \neg \phi\}|$; read as: Few $x$

$^ {\large \unicode{x29bf}} x \, \phi \iff |\{x \mid \phi \}| = |\{x \mid \neg \phi\}|$; read has Half $x$

$^{\large \odot}\kern-0.546em ^{\Large .} x \, \phi \iff ^{\large \odot} x ^{\large \odot} y: x \neq y \land \phi(x) \land \phi(y)$; read as: At least two $x$

$^{\large \odot !} x \iff ^{{\large \odot }^1} x \iff ^{\large \odot} l: {\LARGE \bullet} x \, ( \phi(x) \leftrightarrow x=l)$; read as: there exists a unique $x$

$^{{\large \odot }^n} x \iff ^{\large \odot} l_1,..,^{\large \odot} \! l_n: \overset {\rightarrow} {l_{i \leq n} \neq l_{j \leq n}} \land \\ {\LARGE \bullet} x \, ( \phi(x) \leftrightarrow (x=l_1 \lor..\lor x=l_n))$

;read as: there exists exactly $n$ $x$.

We can also adopt another version of circle notation which is somewhat the opposite of the above, where the white replaces black in the above representation, so for example Choice would be written as:

Choice: $^\bigcirc \! X \ [ ^\bigcirc m, ^\bigcirc \! n \, (m \in X \land n \in X\to \, \, \neg \, ^{\large \odot} l: l \in m \land l \in n) \to \\ ^{\large \odot} C, ^\bigcirc \! x \in X, ^{\large \odot !} \! c: c \in C \land c \in x] $

I find the circles more demonstrative, while the arrows simpler. We can of course modify the arrow notation as to demonstrate the spectrum of quantification concepts, but it would be cumbersome. I think the circle notation is less arbitrary than the arrows. To me both notations seem to be less arbitrary than the customary $\forall, \exists$ language bound notation, and more into capturing aspects of the notion of quantification.

Has any of these notations been attempted before for symbolising quantification?

What are the main drawbacks (other than it not being the convention) for these notations?

Zuhair
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    What are the expected benefits? – Mauro ALLEGRANZA Feb 10 '22 at 09:53
  • @MauroALLEGRANZA, better notational capture of a concept never cease having benefits, at least in firmly engaging with symbolised concept, developing further versions of it, communicating it outside your language, better readbility, etc... – Zuhair Feb 10 '22 at 10:30
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    Language-independent notation is a laudable goal, but the fact that your sample statements use Roman/Greek letters, Arabic numerals, and $\in$ (a stylized $\varepsilon$ w/linguistic origins) suggests that maybe $\forall$ and $\exists$ aren't such a big deal. Usability-wise, the circle notation may be tricky to hand-write effectively ("fills" take time); even type-set, glyph designs (esp. at smaller sizes) may cause trouble visually distinguishing nuances in the "spectrum", especially when comparing different printed works. (Do we need "most/half/few"? They'd be useless w/infinite sets.) – Blue Feb 10 '22 at 10:35
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    To the extent you're interested in what notation may have been used before, and/or motivations for the current standard notation, the History of Science and Mathematics StackExchange may have some insights. See also the "Earliest Uses of Symbols of Set Theory" page (via st-andrews.ac.uk). – Blue Feb 10 '22 at 10:38
  • @Blue, I think pure logical constants are ought to be as much as possible language indepedent, this seems to be the case with $\to, \implies$ , I don't know really about the others. As about $\in$ this is an 'extra-logical' concept, so language dependent terminology is to be allowed, but my point was in freeing logic from language dependent notation. – Zuhair Feb 10 '22 at 11:21
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  • @Blue, my rationale is that Logic is a tool that secures proper streaming of information, it is subject neutral,that is it controls streaming of information of ANY content. That's why its symbols are ought to be flaw control symbols, and moreover simple ones, like one dimentional symbols (Lines), and this conforms with the customary notation of propositional connectives, which can be re-understood in a language free manner. I think quantifiers are to follow the same tradition. Extra-logical concepts are those holding information (denotative), and thus to be represented by letters...cont – Zuhair Feb 10 '22 at 17:14
  • ..continuation: so I personally prefer the arrow notation for quantifiers since its simpler and language free. It depends on More is Up and less is down Metaphor, and then streches it up to for all, and for some. The circles are heavier notation, somewhat concrete. But as you said hard to write practically speaking. But they are very demonstrative and more into what quantification is about than the others. – Zuhair Feb 10 '22 at 17:18
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    The problem with arrows like this is that arrows are already used for so many things in math -- limits, function from domain to range, graph theory, homological algebra diagrams, and a host of other things that I could list if I spent more than 30 seconds with this comment. – Dave L. Renfro Feb 10 '22 at 17:54
  • @DaveL.Renfro, Yes! But the context would make the difference. Most of the arrows you've mentioned are horizontal, and yet that didn't forbid using horizontal arrows in representing implication, also I think the other logical symbols might be heavily used also. Still this won't cause confusion in a context of pure logic; confusion would happen only if we are engaged in logical formalization of those fields, but again I think the context is clear. So, this shouldn't be a big concern. To me logic has priority over all of those. – Zuhair Feb 10 '22 at 18:21

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