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August 3, 2012 / porton

How to teach filters to young mathematicians

I propose the following way to introduce filters on sets to beginning students. (I am writing a book which contains this intro now.) You are welcomed to comment whether this is a good exposition and how to make it even better.

We sometimes want to define something resembling an infinitely small (or infinitely big) set, for example the infinitely small interval near 0 on the real line. Of course there are no such set, just like as there are no natural number which is the difference 2 - 3. To overcome this shortcoming we introduce whole numbers, and 2 - 3 becomes well defined. In the same way to consider things which are like infinitely small (or infinitely big) sets we introduce filters.

An example of a filter is the infinitely small interval near 0 on the real line. To come to infinitely small, we consider all intervals \left( - \varepsilon ; \varepsilon \right) for all \varepsilon > 0. This filter consists of all intervals \left( - \varepsilon ; \varepsilon \right) for all \varepsilon > 0 and also all subsets of \mathbb{R} containing such intervals as subsets. Informally speaking, this is the greatest filter contained in every interval \left( - \varepsilon ; \varepsilon \right) for all \varepsilon > 0.

[A formal definition of a filter on a set goes here.]

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