Here is the link to the Wikipedia article on this.
keywords:
[Mathematics]
[subfactorial of n]
[!n]
[no fixed point]
[e in the denominator]

Here is the link.
This is suggested new terminology.
Here is the link to the explanation of the terminology.
keywords:
[Q & A site about Biology]
[Q & A forum about Biology]

Here is the link.
This is suggested new terminology.
Here is the link to the explanation of the terminology.
keywords:
[Q & A site about Chemistry]
[Q & A forum about Chemistry]

Here is the link.
This is suggested new terminology.
Here is the link to the explanation of the terminology.
keywords:
[Q & A site about Physics]
[Q & A forum about Physics]

Here is the link.
This is suggested new terminology.
Here is the link to the explanation of the terminology.
keywords:
[Q & A site about Mathematics]
[Q & A forum about Mathematics]

(suggested new terminology)
This is the the term I will use for the suite of Question-and-Answer (‘Q & A’) sites known as ‘Stack Exchange’. I will also use a different method for referring to an individual site, namely,
‘Acropolis/Subject’ for ‘Subject Stack Exchange’. So, for example, I will refer to the site ‘Mathematics Stack Exchange’ as ‘Acropolis/Mathematics’.
keywords:
[renaming]
[reference]
[forum]