You don't. So what do think is happening? Has Mendelson said something
monstrously silly, or have you misunderstood what he's saying? Where do
you think any such thing is said, implied or required to be the case?
You keep asking the wrong sort of question. If you think Dr A. N. Umber
"goofed" in his seminal work "How to add" because 2+2 can't possibly be
5 you should not keep asking "how can 2+2 be 5?". You should ask "On
page 1562 he seems to suggest that 2+2 is 5. What have I
misunderstood?".
>> You are not the target audience, so while I agree that he could be
>> clearer if writing for someone like you, it's not a fair criticism of
>> the book because he isn't.
>>
>>> Moreover, instead of talking about the n-tuples of objects that
>>> satisfy a wf that has n free variables, it is much more convenient
>>> from a technical standpoint to deal uniformly with denumerable
>>> sequences [or finite sequences] such that 5-tuple <x27, x3, x99, x39,
>>> x3> would be referred to by a short-hand name such as y3.
>>
>> This, however, is junk. You have no idea how Σ is used so you should
>> not pretend to know how to "be clearer".
>
> I am proposing one very easy way of encoding a sort-hand notation for
> encoding sets of n-tuples.
You should be try to learn how to do it properly. The chances that you
can improve on Mendelson are slim. At any reate, I will ignore your
suggestions until you clearly know what Mendeson is doing. Then you can
tell the world how to do it more clearly from the point of view of
someone who at least knows what "it" is.
>>>>>>> Let Σ be the set of all denumerable sequences of elements of D.
>>>>>>>
>>>>>>> So when D is the set of humans the denumerable sequences of elements
>>>>>>> of D is the empty set,
>>>>>>
>>>>>> I can't see how you get that. A denumerable sequence of humans is an
>>>>>> endless sequence of humans.
>>>>>
>>>>> I thought that a domain in set theory was an entirey different word
>>>>> than the domain from algebra.
>>>>
>>>> It's a very widely used word. Functions have domains. Interpretations
>>>> have domains. There is even a highly technical meaning that is used in
>>>> denotational semantics of programming languages.
>>>>
>>>>> There is no such endless sequence of humans in the whole physical
>>>>> universe.
>>>>
>>>> You can have an endless sequence even if the range has one member.
>>>
>>> The bijection between the natural numbers and the elements of a finite
>>> set breaks when one reaches the size-of-set element of the set.
>>
>> There are no bijections between ℕ and any finite set.
>
> A set X is denumerable if it is equinumerous with the set of positive
> integers.
Sigh. Yes, that's a quote from someone who knows shit.
> A denumerable sequence is a function s whose domain is the set of
> positive integers;
Yup, another quote from someone who knows.
> instead of talking about the n-tuples of objects that satisfy a wf
> that has n free variables, it is much more convenient from a technical
> standpoint to deal uniformly with denumerable sequences.
And still the keep coming... Surely you will get round to saying
something wrong soon.
> When we satify the "father of" relation we do not have a denumerable
> sequence we have a finite sequence therefore his generalization of
We can choose to have a denumerable sequence if we want. You'd have to
read the damn book to see how.
> ...talking about the n-tuples of objects that satisfy a wf that has n
> free variables...
>
> using denumerable sequences is incorrect. In the case of the "father
> of" relation we have a finite sequence of ordered pairs where the
> first element of this pair comes from the set of every father that
> ever lived.
You'd have to read the book to find out how this works. But you've
decided he's wrong (without knowing what Σ is used for) and you can't
get past this block.
>> Why are you
>> stating this rather obvious fact? You must think it relates to my
>> remark "you can have an endless sequence even if the range has one
>> member" but I can't image how.
>
> The bijection does not endlessly continue it stops at one.
That's badly worded. Functions don't "stop". But you are right, there
is no bijection between ℕ and a singleton set. You've said it many
times. I've agreed many times. Yet I still stand by what I wrote (now
cut unfortunately). It was this:
You can have an endless sequence even if the range has one member.
so what do you suppose is going on? Could it be you've just imagined a
problem where one does not exist?
>>> To say that there is a complete bijection between the natural numbers
>>> and the elements of a finite set is psychotic.
>>
>> Yes, that would be daft, wouldn't it? So what should your first thought
>> be? Remember, you want to know what's going on.
>
> Yet another case where the terms of the art make sure to totally screw
> up the common meaning of these same terms. It is like saying that the
> medical term "dead" means {could not be more healthy}.
A sequence is a function from the positive integers to some other set
(called the range of the function). Finite sequences are functions from
{1, 2, ..., n} and infinite sequences (denumerable sequences) are
functions from ℕ to some other set. That other set, the range of the
function, can be a finite set. It can even be a singleton set (though
it can't be empty). Here's an example
s(n) = 42
s(1) is 42, s(2) is 42 and so on. We might write the sequence like
this: (42, 42, 42, ...).
Now I don't really care if you consider some of these words screwy, but
they seem perfectly reasonable to me. More importantly, it's
high-school maths (at least in the UK it is). Everyday I fond you need
to start further back than I thought.
>>>>>>> Let Σ be the set of all denumerable sequences of elements of D.
>>>>> Still makes no sense at all when D is finite.
>>>>
>>>> Can you image a function from ℕ to {true, false}? Maybe the function
>>>> p(n) that is true when n is prime? That is a denumerable sequence of
>>>> truth values. It starts
>>>>
>>>> p_n = (false, true, true, false, true, false, true, false, false, ....)
>>>
>>> No I can't imagine this. p(n) operates on individual constants.
>>
>> If you can't imagine a function from ℕ to {true, false}, you need
>> another hobby. Goodness knows what "constants" have to do with it.
>
> I can imagine a function from two elements of ℕ to each of the
> elements of True and False. I cannot imagine the entire set of ℕ maps
> to the two elements of true and false. I would map false to zero and
> true to 1.
You need a book far more elementary than Mendelson. You've been using
words like "function" as if you know what they mean (heck, you've been
doing that with "set") but you don't. When challenged, you pull a
quote from somewhere, but that just fools people like me into think you
know what the quote means.
>> Remember, constants form a syntactic category in the language of some
>> theory. You are using the word incorrectly for this context, but I
>> don't think you can change.
>
> A function is a relation that...
Oh, talk of the Devil!
> ... uniquely associates members of one set
> with members of another set. More formally, a function from A to B is
> an object f such that every a ∈ A is uniquely associated with an
> object f(a) ∈ B.
https://mathworld.wolfram.com/Function.html
>
> every a ∈ ℕ is uniquely associated with an object f(a) ∈ {true,
> false}.
Do you think that "uniquely associated" means that if, say, f(1) = true,
f(2) can't also be true? Do you think that sin(x) is not a function
because sin(x) = 1 for so many values of x?
--
Ben.