The Universe of Disco

Technical devices for reducing the number of axioms

In a recent article, I wrote:

I guessed it was a mere technical device, similar to the one that we can use to reduce five axioms of group theory to three. …

The fact that you can discard two of the axioms is mildly interesting, but of very little practical value in group theory.

There was a sub-digression, which I removed, about a similar sort of device that does have practical value. Suppose you have a group !!\langle G, \ast \rangle!! with a nonempty subset !!H\subset G!!, and you want to show that !!\langle H, \ast \rangle!! is a subgroup of !!G!!. To do this is it is sufficient to show three things:

1. !!H!! is closed under !!\ast!!
2. !!G!!'s identity element is in !!H!!
3. For each element !!h!! of !!H!!, its inverse !!h^{-1}!! is also in !!H!!

Often, however, it is more convenient to show instead:

For each !!a, b\in H!!, the product !!ab^{-1}!! is also in !!H!!

which takes care of all three at once.

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