NUMEROSITIES OF POINT SETS OVER THE REAL LINE

We consider the possibility of a notion of size for point sets, i e. subsets of the Euclidean spaces E(d)(R) of all d-tuples of real numbers, that satisfies the fifth common notion of Euclid's Elements "the whole is larger than the part". Clearly, such a notion of "numerosity" can agree with cardinality only for finite sets We show that "numerosities" can be assigned to every point set in such a way that the natural Cantorian definitions of the arithmetical operations provide a very good algebraic structure. Contrasting with cardinal arithmetic, numerosities can be taken as (nonnegative) elements of a discretely ordered ring, where sums and products correspond to disjoint unions and Cartesian products, respectively Actually, our numerosities form suitable semirings of hyperintegers of nonstandard Analysis Under mild set-theoretic hypotheses (e g cov(B) = c < N(omega)), we can also have the natural ordering property that, given any two countable point sets, one is equinumerous to a subset of the other Extending this property to uncountable sets seems to be a difficult problem