Le 16-mai-06, à 17:31, Tom Caylor a écrit :

> Bruno Marchal wrote:
>> Now I think I should train you with diagonalization. I give you an
>> exercise: write a program which, if executed, will stop on the biggest
>> possible natural number. Fairy tale version: you meet a fairy who
>> propose you a wish. You ask to be immortal but the fairy replies that
>> she has only finite power. So she can make you living as long as you
>> wish, but she asks precisely how long. It is up too you to describe
>> precisely how long you want to live by writing a program naming that
>> big (but finite) number. You have a limited amount of paper to write
>> your answer, but the fairy is kind enough to give you a little more if
>> you ask.
>> You can ask the question to very little children. The cutest answer I
>> got was "7 + 7 + 7 + 7 + 7" (by a six year old). Why seven? It was the
>> age of his elder brother!
>> Hint: try to generate an infinite set S of more and more growing and
>> (computable) functions, and then try to diagonalize it. S can be
>> {addition, multiplication, exponentiation,  .... (?)....}. More hints
>> and answers later. I let you think a little bit before. (Alas it looks
>> I will be more busy in may than I thought because my (math) students
>> want supplementary lessons this year ...).
>> Hope this can help; feel free to make *any* comments.
>> Remember that if all this is too technical, you can also just read
>> Plotinus and the (neo)platonist which, accepting comp or weaker form 
>> of
>> Pythagorism,  do have a tremendous advance on most materialist of 
>> today
>> ... I think it could even provide more light on the practical death
>> issue. The role of G and G* is just to get the math correct for some
>> notion of quantifying the 1-person probabilities.
>> Bruno
>> (*)SANE paper html:
>> http://iridia.ulb.ac.be/~marchal/publications/SANE2004MARCHAL.htm
>> SANE paper pdf:
>> http://iridia.ulb.ac.be/~marchal/publications/SANE2004MARCHAL.pdf
>> http://iridia.ulb.ac.be/~marchal/
> In keeping with the incremental interactive process, here is a first
> guess.  You simply start naming off the natural numbers in order.
> After naming each number you say, "That's not the largest possible
> natural number", or "That's not how long I want to live."  This
> statement seems to play the role of diagonalization.

But it is not a finite process. The fairy asks you to give a well 
defined number, in a finite time.

> The process I've
> just described can be defined with a finite number of symbols (I just
> did it).  Thus, in a way you can say I've just "named" the largest
> natural number.

You have just given a procedure for building a bigger number from any 
number. The function which send n on n+1  does that trick. But the 
fairy asks you for a number, not a function.

> First question: Is this the same as Douglas Hoftstadter's supernatural
> numbers (in his book Godel, Escher, Bach)?

I have read that quite good book, but I don't have it under the hand, 
and I don't think the big number problem is related to its supernatural 

> It seems the only way to
> really understand his book is to read it cover-to-cover (because of all
> the acronyms and his defining ideas with stories, etc.).  I wish I
> would have read it cover-to-cover when I was young and had lots of time
> on my hands (and lots of spare brain cells).... or may I can just start
> reading it cover-to-cover now and simply ask the fairy for more
> (quality) time as I need it.

Hofstadter wrote a good book, yes, but on the pedagogical side it does 
not help so much by diluting the proof of Godel's theorem in many 
interesting themes (Bach, Escher, AI, etc.).

> Second question:  When we switch over from natural numbers to "length
> of life", it seems we need to specify "units of time" in order for the
> specification of length of life to have any meaning.

You are right. Let us take *years".

>  This crosses us
> over into the realm of meaning.  Length of life has no meaning apart
> from an assignment of meaning or quality to the events that make up
> life.  There seems to be some kind of diagonalization going on here (or
> perhaps transcendence, independent from any diagonalization argument).
> What good is MWI "immortality" (or any kind of "immortality") if the
> infinite sum of (units of time) * (quality or meaning) adds up to some
> finite number?  Is it really immortality? Life is more than existence.

In the big number problem, immortality is not proposed by the fairy, 
what is proposed is just a long but finite life.
Here too the quality is important. To stay a very long time awake in a 
coffin is not pleasant. Also, to stay alive for a very long period 
makes almost no sense if your brain is limited in space (bounded finite 
machine eventually cycle when running a long time. Do you see why?).

The big number problem has been tackled by Archimedes. He got the 
number 10^63. This is remarkable if you recall the very bad notation 
for number used at that time. Today 10^63, although very big (the 
universe seems to exist since less than 10^17 sec and I think that 
number has shrinked recently) seems rather little. It is little than a 
Googol (10^100) and a Googolplex is far bigger: 10^[a googol]. But 
really we will end up with far bigger (but still finite) number once we 
will diagonalise on set of growing functions (soon).

Meanwhile just a few questions to help me. They are hints for the 
problem too. Are you familiar with the following "recursive" program 
for computing the factorial function?

fact(0) = 1
fact (n) = n * fact(n - 1)

Could you compute "fact 5", from that program? Could you find a similar 
recursive definition (program) for multiplication (assuming your 
machine already know how to add)?
Could you define exponentiation from multiplication in a similar way?  
Could you find a function which would grow more quickly than 
exponentiation and which would be defined from exponentiation like 
exponentiation is defined from multiplication? Could you generalize all 
this and define a sequence of more and more growing functions. Could 
you then diagonalise effectively (= writing a program who does the 
diagonalization) that sequence of growing functions so as to get a 
function which grows more quickly than any such one in the preceding 

I have no precise idea of your background, so ask if there is anything 
unclear. Anyone can ask.


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