Re: wff's definition

David C. Ullrich wrote:
On 1 Dec 2006 01:07:14 -0800, "pukun" <saurav1111@xxxxxxxxxxxxxx>
wrote:


In almost every book, including Mendelson's, the definition of wff
doesn't seem satisfactory. First, it should have been defined as class
of members of a known well defined set, say T, and by axiom of subset,
it should have been CONCRETELY settled whether a particular member of T
is a wff or not.


It's not clear whether your complaint is (a) the definition is
not formally justified using the axioms of ZFC or (b) you're
concerned that the definition does not actually specify what
is and what is not a wff.

If (a): That's a silly objection; nobody said that the
definition _was_ done in ZFC.

David is quite right. When I read the book by Mendelson (admittedly about 20 years ago and probably an edition completely out of print), wff's are defined in terms of naive set theory. ZFC (or NBG in his case) is defined in a much later chapter, in terms of 1st order logic, necessitating wff's, so to do it as you want to do it is circular reasoning.

As long as classes have been cancelled

Same here. I guess Oklahoma got the same snow storm as Missouri.

and I still
have no clue about today's question on Dini derivatives:

Better than me. I don't even remember what a Dini derivative is!! (Is it something I taught in "Real Variables," a class I haven't touched for 15 years?)

You can use the same outline to construct the parse
tree.

When I went through my "new computer language learning phase" I went for Perl instead of Python.

Stephen
.

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