Re: symbols/codepoints for necessity and possibility in modal logic

On Thursday 18 July 2013 22:27:52 Stephan Stiller wrote:
> Hi all,
>
> Modal logic uses a "box" and a "diamond" (this is how they're informally
> called) as operators (accepting one formula and returning another) to
> denote necessity and possibility, resp. Older texts might use the
> letters L and M (resp). Which Unicode codepoints do modal box and
> diamond correspond to?
>
> According to the charts, it seems like the box is
> ◻ (U+25FB)
> (is this definitive?), but what about the diamond? Unlike what one might
> glean from the charts, ⟠ (U+27E0) is afaiu /not/ normally used to denote
> possibility in the default† sense. Wiki's "List of logic symbols"
> article has something to say about this too, but I'm always cautious
> about information from there.
        There are a number of "box" characters in the vicinity of U+27FB that might
be used: U+25A1, U+25AB, U+25AF (doubtful), and U+25FD. For diamond, a
better choice might be U+25CA, which does not have the horizontal bar.
        At http://plato.stanford.edu/entries/logic-modal/ I see that the web page
uses U+25A1 for box, and U+25CA (aka ◊ by w3c) for diamond.

Leslie
Received on Fri Aug 02 2013 - 09:22:21 CDT