# Finding the units (number theory)

## Homework Statement

Prove that the ring R of polynomials with real coefficients (i.e. f(x) = a0 + a1x + ... + anxn, ai real, are elements of R) has only the constant term a0 as the group of units, providing the constant term isn't zero.

## Homework Equations

u is a unit if there exists a v such that uv=1

## The Attempt at a Solution

Clearly all the constant terms are units of the ring as we are dealing with the real numbers.

Claim: There are more than just these, and that a polynomial f(x) = a0 + a1x + ... + anxn is invertible.

=> there must exist a g(x) = b0 + b1x + ... + bnxn

such that f(x).g(x) = 1

f(x).g(x) = b0(a0 + a1x + ...) + b1x(a0 + a1x + ...) + b2x2(a0 + a1x + ...) + ... = 1

Not too sure where to go from here.. Sorry if my notation is a little off but hopefully you can understand

Thanks

## Answers and Replies

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R[x] is the ring of polynomials over a field: R. In this case, what is the degree of the product of two polynomials? What must be the degree of a unit?

Dick
Homework Helper
If f(x) and g(x) aren't constants, then each one has a highest degree term with a coefficient that's nonzero, right? What's the coefficient of the highest degree term of f(x)*g(x)?

R[x] is the ring of polynomials over a field: R. In this case, what is the degree of the product of two polynomials? What must be the degree of a unit?
If f has degree n and g has degree m then the degree of the product is n+m which must be greater than or equal to 2, but the the degree of 1 is 0.

Surely some polynomial can equal 1 though can't it?

If f(x) and g(x) aren't constants, then each one has a highest degree term with a coefficient that's nonzero, right? What's the coefficient of the highest degree term of f(x)*g(x)?
coefficient anbm, not too sure how this helps though

Dick
Homework Helper
If an*bm is not zero, then f(x)*g(x) contains a term of the form an*bm*x^(n+m) and that's the only term of that degree. So the product is probably not 1.

But if these are real coefficients then maybe they're small enough so we can choose an x such that the product is equal to 1?

Dick
Homework Helper
"1" has coefficient 0 for all powers of x greater than 0. Not "close to zero". Zero. I'm having a hard time picturing what you are thinking about here. The reals don't have any zero divisors.

Dick
Homework Helper
But if these are real coefficients then maybe they're small enough so we can choose an x such that the product is equal to 1?
"x" isn't a number. It's a symbol. This question isn't about roots of a polynomial. Is that what you are thinking?

"x" isn't a number. It's a symbol. This question isn't about roots of a polynomial. Is that what you are thinking?
ye i was thinking that, guess that's why I'm having a hard time understanding

I can just go by the degrees of 1 and the product of the polynomials are different then as above?

Thanks

Dick