# Continuity in algebra?

1. Sep 21, 2007

### pivoxa15

Does the notion of continuity exist in modern algebra?

If so how do they arise?

2. Sep 21, 2007

### Hurkyl

Staff Emeritus
You can certainly have topological monoids, groups, rings, et cetera. And you can construct topological spaces related to algebraic structures.

3. Sep 21, 2007

### mathwonk

see zariski topology, and the metric completion of a ring wrt an ideal.

4. Sep 21, 2007

### pivoxa15

So it can happen in algebra but rare for pure algebra as opposed to algebra mixed with other maths disciplines like topology.

But would you say 'discreteness' plays a large role in modern algebra?

How would it sound if someone said they like modern algebra because of its discreteness which lacks in say analysis and topology?

Last edited: Sep 21, 2007
5. Sep 22, 2007

### Hurkyl

Staff Emeritus
I would think you meant something more like "exact" than "discrete". I'm not sure what you consider "pure", but commutative algebra, at least, is very closely intertwined with algebraic geometry.

6. Sep 22, 2007

### mathwonk

define as metric on a ring wrt a fixed maximal ideal m, where the distasnce between f and g is 1/2^n, where n the largest n such that f-g belongs to m^n.

then in this metric, where m = the maximal ideal of the origin in the polynomial ring, a formal power series is the limit of the sequence of its partial sums. i.e. one can complete a polynomial ring wrt a maximal ideal, and get a power series ring.

this leads to the concept of the completion of a ring wrt an ideal. this concept helps recover local concepts in algebra,

"more local" than given by the zariski topology.

7. Sep 22, 2007

### matt grime

How many angels can dance on the head of a pin? More semantic hair splitting. Go, on, pixova, define 'pure algebra'.....

8. Sep 22, 2007

### mathwonk

a basic philosophy is that to understand a thing, one wants to construct as many invariants of it as possible. so to understand a ring, one can consider obvious invariants such as its cardinality, or its group of units.

but one can also look at more complicated invariants such as the associated affine scheme, or in the case of a local ring, its completion wrt the metric defined above.

for the local ring of a smooth point on an algebraic variety this leads to a proof of the theorem in pure algebra that the ring is a ufd because its completion is so.

9. Sep 22, 2007

### HallsofIvy

I would be inclined to say that all of those many situations in which you have both continuity (i.e. topology) and an algebraic structure are "analysis" rather than just algebra. In my (probably idiosycratic) opinion, analysis is the combination of algebra with topology.

10. Sep 22, 2007

### pivoxa15

So is that why Herman Weyl said something like "...algebra and topology fight for every branch of mathematics." So even a fundalmental discipline like analysis is a combo of algebra and topology.

11. Sep 22, 2007

### pivoxa15

I can't define pure algebra just as I can't define mathematics. But I can give some examples. Galois theory is pure algebra. Whereas topics in algebraic topology is not pure algebra. There may also me things in between.

Last edited: Sep 22, 2007
12. Sep 22, 2007

### pivoxa15

What do you mean by invariants? Never changing?

http://en.wiktionary.org/wiki/invariant

Do you mean its properties?

13. Sep 23, 2007

### mathwonk

An invariant is almost another name for a functor. I.e. an object or quantity F(X) constructed from X is an invariant of [the isomorphism type of] X if F(X) and F(Y) are isomorphic or equal whenever X and Y are.

14. Sep 23, 2007

### matt grime

Examples: (co)homology rings, fundamental groups, etc of topological spaces are invariant under homeomorphism (and homotopy).

The ultimate aim of mathematics for some people is to find a set of invariants that uniquely characterise objects up to whatever level of equivalence you are working with. See, e.g. compact surfaces, genus, homotopy, and the poincare conjecture, for example.

15. Sep 23, 2007

### Hurkyl

Staff Emeritus
The Galois group is profinite, its topology plays a criticial role in the theory.

Finite profinite groups are discrete -- if you've only worked with finite Galois extensions, that would explain why you haven't seen this before. A useful example of an infinite Galois extension is $\mathbf{\bar{Q}} / \mathbf{Q}$: the algebraic numbers over the rationals.

16. Sep 23, 2007

### matt grime

Why not? Yes, I am playing devil's advocate, but I'd like you to examine your own preconceptions about mathematics.

17. Sep 23, 2007

### mathwonk

i believe herman weyl actually said more like: " the angel of topology and the devil of abstract algebra fight for the soul of every discipline in mathematics."

18. Sep 23, 2007

### HallsofIvy

Oh, I like that!

Does the fact that he said "angel of topology" and "devil of abstract algebra" tell us that he was a topologist?

19. Sep 23, 2007

### mathwonk

read some of weyls books. he is author of some outstanding algebra books. as well as books on riemann surfaces,...