Finding an English translation to Kazimierz Kuratowski’s finiteness paper.

elementary-set-theoryreference-request

Dear Maths Stackexchange.

I am currently doing some reading into the different notions of how finiteness is modelled in set theory. One formulation that took my eye was Kazimierz Kuratowski's formulation which seems to me rather elegant. Whist I have no problem with understanding the mathematics of this formulation, I am nonetheless interested in viewing the original source paper for this construction.

The earliest text I can find is given in this short three page paper here in French. Unfortunately I cannot read French, so this is of little use to me in the given state. I have done some poking around on google and google scholar and come up blank when looking for translations of said paper into English. Thus I open the following question up with the hopes that someone knows the location of an appropriate reference:

Is there an English source to this paper?

Best Answer

Taking the advice of @Rob Arthan, I digitised the text, corrected any errors in the digitisation before passing this text through to google translate. I then attempted to correct a couple of errors made by the machine in translation. The finished English text seems accurate to my sensibilities, however I am not in a position to give strong guarantees on the accuracy of my translation. I have been as conservative as possible in any edits I have made. Below I present the text, formatted with the usual Math-Stackechange formatting tools:


On The Notion of a Finite Set.

by Casimir Kuratowski (Warsaw).

Mr. W. Sierpiński gave in his work The axiom of M. Zermelo and its role in Set Theory and Analysis${}^1$ a new definition of the finite set. This definition is mainly distinguished by the fact that it depends neither on the notion of natural number nor on the general notion of function, which usually falls within the definitions making use of the notion of correspondence. The definition in question is as follows:

Consider class $K$ of sets each of which satisfies the following conditions:

  1. Any set containing a single element is part of the class K.
  2. If$A$ and $B$ are two sets belonging to the class $K$, their sum-set $A + B$ also belongs to $K$.

Let us call finite any set which belongs to each of the classes K satisfying the conditions 1 and 2.

As we know, the set of all. objects (if it exists) has paradoxical properties: contrary to a theorem known by G. Cantor, the power of this set would not be lower than that of the class of all its subsets. It is the same for the class composed of all the sets containing a single element; therefore, the classes K do not satisfy Cantor's theorem. Taking this fact into account, one could question the very existence of $K$-classes.

By modifying Mr. Sierpiński's definition so as to eliminate this drawback, I obtain the following definition:

The set $M$ is finite, when the class of all its (non-empty) subsets is the unique class satisfying the conditions:

  1. Its elements are (nonempty) subsets of $M$.

  2. Any set containing a single element of $M$ belongs to this class.

  3. If $A$ and $B$ are two sets belonging to this class, their sum-set $A + B$ also belongs to it.

We are going to demonstrate that a finite set according to this definition is also in the ordinary sense and vice versa. In other words: for a set to be finite according to the proposed definition, it is necessary and sufficient that the number of its elements can be expressed by a natural number (the notion of a natural number being assumed to be known).

Indeed, let $M$ be a set whose number of elements can be expressed by a natural number; let Z be any class satisfying conditions 1—3. We will show that every subset of $M$ belongs to $Z$. This is the case - by virtue of condition 2 - with subsets composed of a single element; at the same time, if this is the case with subsets containing $n$ elements, it is the same - according to 3 - with those which contain $n + 1$. As the number of elements of each subset of $M$ can be expressed by a natural number, it follows by induction that $Z$ contains all the subsets of $M$. Therefore, the class $Z$ being necessarily identical to that of all the subsets of $W$, it is the only class satisfying conditions 1—3. Thus, any set whose number of elements can be expressed by a natural number is a finite set in our sense.

Suppose, on the other hand, that the number of elements of a given set $M$ cannot be expressed by a natural number. Let us denote by $Z$ the class of all the subsets of $M$ whose number of elements can be expressed by a natural number. This class obviously satisfies conditions 1—3; at the same time, according to the hypothesis, $M $ does not belong to $Z$ and, therefore, $Z$ is not identical to the class of all the subsets of M; therefore, the class of all the subsets of $M$ is not the only class satisfying conditions 1 - 3 and is not finite in our sense, Q.E.D.

${}^1$ Bull. of Acad. des Sciences de Cracow, 1918, p. 106.


I have made the answer community so that if anyone confident in both English and French wants to either increase the accuracy of this translation, co-sign the accuracy of translation, or otherwise condemn it, they are free to do so.

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