# More number theory

For i) I said for a in [0,1], then the group of units are = {f in R | f(a) =/= 0}

i.e a continous function f on [0,1] would have a continous function g on [0,1] such that f.g=1

but the function would have to be g = 1/f, but this wouldnt be continous if f(a) = 0

for ii) I have to show for a,b in R then if ab=0 then either a = 0 or b = 0

but I havn't gotton any further here since I'm having a hard time finding functions a and b where the product is 0

for iii) I believe I have to construct a surjective ring homomorphism ( f: R -> F ) and show that the kernal is equal to the Ideal.

But I'm not sure how what field F to choose for this

Or I have to show that the quotient group R/I is a field.

Thanks for any help!

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Dick
Homework Helper
The statement of (i) doesn't have any 'a' in it. You aren't thinking very abstractly. Yes, the inverse of f would be 1/f. What conditions on f do you need to ensure that exists and is continuous?

The statement of (i) doesn't have any 'a' in it. You aren't thinking very abstractly. Yes, the inverse of f would be 1/f. What conditions on f do you need to ensure that exists and is continuous?

the condition that f is a polynomial?

Dick
Homework Helper
the condition that f is a polynomial?

Is that just a random guess?

Is that just a random guess?

No it wasn't, I looked up a more precise definition of the conditions for continuity and it stated that all polynomials are continuous.

Otherwise I'm not sure what you mean sorry :/

Dick
Homework Helper
No it wasn't, I looked up a more precise definition of the conditions for continuity and it stated that all polynomials are continuous.

Otherwise I'm not sure what you mean sorry :/

Ok. Maybe I'm just being overly fussy about your original answer because you didn't specify what 'a' is supposed to be. Is it some point in [0,1], or is it ANY point in [0,1]?

Ok. Maybe I'm just being overly fussy about your original answer because you didn't specify what 'a' is supposed to be. Is it some point in [0,1], or is it ANY point in [0,1]?

Any point.

Dick
Homework Helper
Any point.

Ok. Be careful when you get to part (iii). There 'a' means 'some point'. So f is a nonzero continuous function on [0,1]. So 1/f exists. If you can say why 1/f is continuous then you've show f is a unit.

For part iii) I've said

that I is a proper ideal since the constant function f(x)=1 is not an element of the ideal.

So R/I is isomorphic to the image of the functions, and the Im(f) = the real numbers

Since the real numbers are a field then I is maximal. Is this ok?

Dick
Homework Helper
It's not very clear. What exactly is the ring homomorphism from R (the ring) to F (the reals) that I is the kernel of?

It's not very clear. What exactly is the ring homomorphism from R (the ring) to F (the reals) that I is the kernel of?

phi : R -> R

f -> f(a)=0

Pretty sure that's a ring homomorphism

Hurkyl
Staff Emeritus
Gold Member
"f(a)=0" is a truth-value (well, a truth-value-valued equation) not a real number....

"f(a)=0" is a truth-value (well, a truth-value-valued equation) not a real number....

hmm so what could I change it to, just 0? Or do I need to change my co-domain?

Dick
Homework Helper
hmm so what could I change it to, just 0? Or do I need to change my co-domain?

You just aren't expressing yourself very well. What is the real number phi(f)?

You just aren't expressing yourself very well. What is the real number phi(f)?

I'm having a hard time getting my head around the ring of functions, usually it's something simpler.

Perhaps it's just

phi : R -> R

f -> f(a)

Dick
Homework Helper
Yes, it is. Is I the kernel? You said before you were only 'pretty sure' that it was a homomorphism. Can you check that it satisfies the properties of a homomorphism, so you can say you are 'really sure'?

For part ii) I can only think of the fact that

'Theorem: Let R be an integral domain and let R[x] be the ring of polynomials in powers of x whose coefficients are elements of R. Then R[x] is an integral domain if and only if R is.'

These polynomials R[x] are functions that are most likely continous on [0,1], so if I can choose an R that isn't an integral domain then by the theorem the continuous functions on [0,1] are not an integral domain?

Just a thoguht as I'm having trouble with this..

Dick
Homework Helper
The problem is not about polynomials, it's about continuous functions, which don't have to be polynomials. Surely you can think of two nonzero continuous functions on [0,1] whose product is zero.

You make it sound trivial but nothings jumping out at me

Dick
Homework Helper
You make it sound trivial but nothings jumping out at me

How about defining the functions piecewise?

hmm if they're piecewise then how to make sure they both stay continuous on [0,1] so that the product is zero?

is it still continuous if the endpoints match up?

so f(x) = x for x<0.5

and g(x) = x for x>=0.5

is sufficient?

Dick
Homework Helper
You haven't defined what f(x) is for x>=0.5. If you mean f(x)=0 for x>=0.5, then f(x) isn't continuous. Making sure the pieces match up so it's continuous is YOUR job.

You haven't defined what f(x) is for x>=0.5. If you mean f(x)=0 for x>=0.5, then f(x) isn't continuous. Making sure the pieces match up so it's continuous is YOUR job.

how about for some small epsilon e > 0

f(x) = e for x<0.5 and f(x) = 0 for x>=0.5, would that be continuous?

Dick
Homework Helper
how about for some small epsilon e > 0

f(x) = e for x<0.5 and f(x) = 0 for x>=0.5, would that be continuous?

What do you think? How small would e have to be to make it continuous? This isn't supposed to be anything horribly exotic. Just pick a nonzero function on [0,.5] such that f(.5)=0.

What do you think? How small would e have to be to make it continuous? This isn't supposed to be anything horribly exotic. Just pick a nonzero function on [0,.5] such that f(.5)=0.

oh so something like f(x) = -x+0.5 for x<0.5, f(x)=0 for x>=0.5

g(x) = x - 0.5 for x>0.5, g(x) = 0 for x<=0.5