We consider the possibility of a notion of size for point sets, i.e. subsets of the Euclidean spaces 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 < ℵω), 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.

### Numerosities of point sets over the real line

#### Abstract

We consider the possibility of a notion of size for point sets, i.e. subsets of the Euclidean spaces 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 < ℵω), 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.
##### Scheda breve Scheda completa Scheda completa (DC)
2010
DI NASSO, Mauro; Forti, Marco
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Utilizza questo identificativo per citare o creare un link a questo documento: `https://hdl.handle.net/11568/195326`
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