[FOM] Towards a Real Finitism?
Karlis Podnieks
Karlis.Podnieks at mii.lu.lv
Thu Dec 29 11:11:07 EST 2005
[Inspired in part by reading Doron Zeilberger's Opinion 57 -
http://www.math.rutgers.edu/~zeilberg/Opinion57.html.]
[For a better formatted version of this message see
http://www.ltn.lv/~podnieks/finitism.htm.]
We see a minority among mathematicians who do not accept the actual infinity
while accepting the potential one. This resembles to me "platonism with
respect to natural numbers" as opposed to unrestricted set-theoretical
platonism. Still, should "subtle differences" play an important role in
philosophy? Shouldn't a useful philosophy be "robust"? So, why should we
stop at rejecting only one kind of infinity?
In this way some investigators have already arrived at various forms of the
so-called ULTRA-FINITISM. But isn't their starting point mainly negative -
rejecting infinity? A kind of positive product could be the notion of
feasible natural numbers. But, is this structure rich enough to become
really useful?
Shouldn't a real finitism be more radical and starting positively? Couldn't
we obtain a richer structure by replacing the traditional "linear" notion of
natural numbers by the notion of FUNCTIONAL EXPRESSIONS?
Of course, this idea is not new - its history may be traced back to 1934 -
to Bernays' number 67^257^729 (where ^ stands for exponentiation): "What
does it mean to claim the existence of an Arabic numeral for the foregoing
number, since in practice we are not in a position to obtain it?"
But what, if we would go further and claim that EXPRESSIONS ARE MORE
FUNDAMENTAL THAN NATURAL NUMBERS? And that natural numbers are simply
expressions based on 1 and +, i.e. 1, 1+1, 1+1+1 etc.? Binary numerals can
be regarded then as expressions based on 1 and two functions - 2x and 2x+1.
And, in general, a "functional base" could consist of any small list of
natural numbers and any small list of computable arithmetical functions. Of
course, to complete this argument consistently, we should completely remove
the traditional natural numbers "from behind"...
But, as a first approximation, let us still use the traditional numbers -
simply to allow comparing of expressions by their natural number "values".
Imagine, we have two different functional bases (for example, {1, +, *, ^}
and {1, +, *}), how complicated is the task of converting expressions of
Base 1 into equivalent expressions of Base 2? And how complicated is the
task of comparing the "values" of two expressions belonging to the same
functional base? As Bernays' example shows, the conversion task may be
unfeasible. But then, how complicated could be the task of comparing two
feasible expressions having unfeasible "values"?
As the first step, I would propose the following 3 problems:
PROBLEM 1. How complicated is comparing the values of two expressions based
on {1, +, *}, the longer expression having the length n?
PROBLEM 2. The same for expressions of base {1, +, *, ^}.
PROBLEM 3. Build a functional base for which the complexity of Problem 1
belongs to a given complexity class.
Are the solutions of these problems already known? Are they trivial? I was
not able to find the answer on the Internet. But the idea was (at least)
touched on the FOM list in October 1998:
by Harvey Friedman: "THEOREM 3. n(3) > A_7198(158386)." (see
http://www.cs.nyu.edu/pipermail/fom/1998-October/002339.html);
by Anatoly Vorobey: "You didn't, after all, prove that n(3)=A100(200); but
you did prove that n(3)>A7(184). What does *that* mean?... So, a
finitist/feasabilist may ask, what kind of information, insight, knowledge,
does n(3)>A7(184) offer you?" (see
http://www.cs.nyu.edu/pipermail/fom/1998-October/002367.html);
and by Vladimir Kanovei: "Most likely 9^9^9... (8 times) < 8^8^8...(9
times). If so what does it mean?" (see
http://www.cs.nyu.edu/pipermail/fom/1998-October/002370.html).
See also web-pages by Robert P. Munafo: "We cannot compute the exact values
of these two numbers and compare directly - they have way too many digits to
store the values on a computer."
(http://home.earthlink.net/~mrob/pub/math/largenum.html)
and a posting of Adam: "Comparing Very Large Numbers--Which Is Bigger?...
How do you know whether 10^10!^10 is bigger than 9!^9!^9?"
(http://mathforum.org/library/drmath/view/66156.html).
But the very first could have been J. E. Littlewood (sorry, my own
translation back from Russian): "The reader will agree that we have written
large numbers; however, it's hard to imagine how large they are; all that
can be said about them - that they are determined by their definitions. If
we would need comparing of two numbers from two competing systems, then
creating of a serious mathematical aparatus may be needed." (see his paper
"Large Numbers", Mathematical Gazette, 32 #300, July 1948. Reprinted in
LITTLEWOOD'S MISCELLANY.)
Karlis Podnieks
Institute of Mathematics and Computer Science
University of Latvia
www.ltn.lv/~podnieks
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