Varieties, rings, fields

CRGreathouse

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A commutative ring is a variety, because its definition consists only of universally quantified identities:
g+(h+k) = (g+h)+k
g+0=g
(-g) + g = 0
g + h = h + g
g(hk) = (gh)k
g(h+k) = gh+gk
1g = g
gh = hg
where (-g) denotes the additive inverse of g.

Adding a new predicate symbol "(1/...)" defined by
(1/g)g = 1

and adding this identity to the list, a new variety is created, whose members are fields along with the trivial ring.

Why is it so important to exclude the trivial ring in the definition of a field (by the requirement [itex]0\neq1[/itex] or [itex]|G|\ge2[/itex]) even though fields are not varieties? Is there any value to the structure defined above (the smallest variety containing all rings)?
 

matt grime

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Um, isn't a variety an algebraic subset of P^n or A^n?
 

CRGreathouse

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Different kind of variety. I'm talking about the sense in universal algebra:
http://en.wikipedia.org/wiki/Variety_(universal_algebra [Broken]) (I'm using Wikipedia to help me through this PDF: http://www.math.iastate.edu/cliff/BurrisSanka.pdf [Broken])
 
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Hurkyl

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...

Adding a new predicate symbol "(1/...)" defined by
(1/g)g = 1

and adding this identity to the list, a new variety is created, whose members are fields along with the trivial ring.
Not true. Adding this gives you the degenerate variety: your axioms allow you to prove the identity x=0 as follows:

0 = (x (1/0))0 = x ((1/0) 0) = x 1 = 1 x = x.


Fields are not a universal algebra. This can be seen, for example, by noting that the Cartesian product of two fields is not a field.
 

CRGreathouse

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Ah, right. That's what I was looking for, thanks!
 

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