Empty set

From: Jukka K. Korpela <jkorpela_at_cs.tut.fi>
Date: Thu, 12 Sep 2013 21:06:54 +0300

Under Subject: Re: Why blackletter letters?
2013-09-12 20:20, Stephan Stiller wrote:

> Talking about which ...
>> I confess I usually type a Danish Ø for convenience when I'm using
>> this, though for publication I would tend to substitute the proper ∅.
> Whenever I saw the empty set symbol in printed math literature in
> Germany, it closely resembled Ø; I don't think I ever saw a
> struck-through circle ∅.

This might be explained by the fact that the letter “Ø” has been
available in fonts (including modern digital fonts) much more often than
the empty set symbol. In any case, the standard ISO 80000-2
unambiguously defines the empty set symbol as U+2205.

> And below the university level Germans
> write { }, which I like better.

The notation { } is quite correct. It just isn’t an atomic symbol for
the empty set but an expression consisting of the two characters “{” and
“}”, with a list (here, an empty list) of elements between them.

> I suspect that circular ∅ and ∖ at 45° are more recent symbols,
> intentionally designed to look different from Ø and \. Does anyone know?

Regarding the empty set, the page
rather convincingly attributes the symbol to André Weil, who says that
it was inspired by the Norwegian letter “Ø”.

Received on Thu Sep 12 2013 - 13:08:37 CDT

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