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https://arxiv.org/pdf/1212.5740.pdf
Filters and Ultrafilters in Real Analysis 2012
Max Garcia Mathematics Department California Polytechnic State University

Abstract
We study free filters and their maximal extensions on the set of natural numbers.
We characterize the limit of a sequence of real numbers in terms of the Fr´echet filter, which involves only one quantifier as opposed to the three non-commuting quantifiers in the usual definition.
We construct the field of real non-standard numbers and study their properties.
We characterize the limit of a sequence of real numbers in terms of non-standard numbers which only requires a single quantifier as well.
We are trying to make the point that the involvement of filters and/or non-standard numbers leads to a reduction in the number of quantifiers and hence, simplification, compared to the more traditional ε, δ-definition of limits in real analysis.

Contents
Introduction . . 1
1 Filters, Free Filters and Ultrafilters 3
1.1 Filters and Ultrafilters . . .. 3
1.2 Existence of Free Ultrafilters . . . . . . 5
1.3 Characterization of the Ultrafilter . . . . . . 6
2 The Fr´echet Filter in Real Analysis 8
2.1 Fr´echet Filter . . . . . . . . . 8
2.2 Reduction in the Number of Quantifiers . . .. . . 10
2.3 Fr´echet filter in Real Analysis . . . . . . . 11
2.4 Remarks Regarding the Fr´echet Filter . . . . . 12
3 Non-standard Analysis 14
3.1 Construction of the Hyperreals *R . . . . . 14
3.2 Finite, Infinitesimal, and Infinitely Large Numbers . . . . . . . 16
3.3 Extending Sets and Functions in *R . . . . . . . . . . . . . . . 20
3.4 Non-Standard Characterization of Limits in R . . . . . . . . . 23
A The Free Ultrafilter as an Additive Measure 25