3,672 research outputs found
Cauchy completeness in elementary logic
The inverse of the distance between two structures A not equal B of finite type sis naturally measured by the smallest integer q such that a sentence of quantifier rank q-1 is satisfied by A but not by B. In this way the space Str of structures of type tau is equipped with a pseudometric. The induced topology coincides with the elementary topology of Str(tau). Using the rudiments of the theory of uniform spaces, in this elementary note we prove the convergence of every Cauchy net of structures, for any type tau.6141153115
Tameness in generalized metric structures
We broaden the framework of metric abstract elementary classes (mAECs) in
several essential ways, chiefly by allowing the metric to take values in a
well-behaved quantale. As a proof of concept we show that the result of Boney
and Zambrano on (metric) tameness under a large cardinal assumption holds in
this more general context. We briefly consider a further generalization to
partial metric spaces, and hint at connections to classes of fuzzy structures,
and structures on sheaves
On the Cauchy Completeness of the Constructive Cauchy Reals
It is consistent with constructive set theory (without Countable Choice,
clearly) that the Cauchy reals (equivalence classes of Cauchy sequences of
rationals) are not Cauchy complete. Related results are also shown, such as
that a Cauchy sequence of rationals may not have a modulus of convergence, and
that a Cauchy sequence of Cauchy sequences may not converge to a Cauchy
sequence, among others
The reals as rational Cauchy filters
We present a detailed and elementary construction of the real numbers from
the rational numbers a la Bourbaki. The real numbers are defined to be the set
of all minimal Cauchy filters in (where the Cauchy condition is
defined in terms of the absolute value function on ) and are proven
directly, without employing any of the techniques of uniform spaces, to form a
complete ordered field. The construction can be seen as a variant of Bachmann's
construction by means of nested rational intervals, allowing for a canonical
choice of representatives
Cauchy, infinitesimals and ghosts of departed quantifiers
Procedures relying on infinitesimals in Leibniz, Euler and Cauchy have been
interpreted in both a Weierstrassian and Robinson's frameworks. The latter
provides closer proxies for the procedures of the classical masters. Thus,
Leibniz's distinction between assignable and inassignable numbers finds a proxy
in the distinction between standard and nonstandard numbers in Robinson's
framework, while Leibniz's law of homogeneity with the implied notion of
equality up to negligible terms finds a mathematical formalisation in terms of
standard part. It is hard to provide parallel formalisations in a
Weierstrassian framework but scholars since Ishiguro have engaged in a quest
for ghosts of departed quantifiers to provide a Weierstrassian account for
Leibniz's infinitesimals. Euler similarly had notions of equality up to
negligible terms, of which he distinguished two types: geometric and
arithmetic. Euler routinely used product decompositions into a specific
infinite number of factors, and used the binomial formula with an infinite
exponent. Such procedures have immediate hyperfinite analogues in Robinson's
framework, while in a Weierstrassian framework they can only be reinterpreted
by means of paraphrases departing significantly from Euler's own presentation.
Cauchy gives lucid definitions of continuity in terms of infinitesimals that
find ready formalisations in Robinson's framework but scholars working in a
Weierstrassian framework bend over backwards either to claim that Cauchy was
vague or to engage in a quest for ghosts of departed quantifiers in his work.
Cauchy's procedures in the context of his 1853 sum theorem (for series of
continuous functions) are more readily understood from the viewpoint of
Robinson's framework, where one can exploit tools such as the pointwise
definition of the concept of uniform convergence.
Keywords: historiography; infinitesimal; Latin model; butterfly modelComment: 45 pages, published in Mat. Stu
Stevin numbers and reality
We explore the potential of Simon Stevin's numbers, obscured by shifting
foundational biases and by 19th century developments in the arithmetisation of
analysis.Comment: 22 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1104.0375, arXiv:1108.2885, arXiv:1108.420
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