Homework Help: Finding coefficients for reducibility (Abstract Algebra)

1. Feb 13, 2016

RJLiberator

1. The problem statement, all variables and given/known data
Find all real numbers k such that x^2+kx+k is reducible in ℝ[x].

2. Relevant equations

3. The attempt at a solution
This seems like it is simple, but it is new to me so I am looking for confirmation.

We know we can find the roots of a polynomial with b^2-4ab. We want b^2-4ab to be greater than 0 for it to have roots.

But we note:

Here, a = 1, b = b, c = b.

When we have b^2-4*b = 0 we get the answers either b = 0 or b = 4.
Both of these are solutions to the problem question.

If we let b(b-4) > 0 then we get a whole array of numbers that are not solutions to this problem.

So I am stating that b =0 and b = 4 are the only solutions to the problem as they are the only solutions to b^2-4ab = 0. But why is this so?

2. Feb 13, 2016

Staff: Mentor

Greater or equal 0. Only negative solutions will lead to negative square roots.
Right.
Why? What happens if b=6? (I assume b=k.)
See previous comment. Plus: what happens if b=2?

3. Feb 13, 2016

RJLiberator

Fresh_42, If we have b = 6
then we have:
x^2+6x+6

This is not reducible under the real numbers, am I right?

4. Feb 13, 2016

Staff: Mentor

No. You can see it if you draw the function, it crosses the x-axis twice which means it is 0 at these points. What would be the solutions? What has $b^2-4ab$ to do with the roots?

5. Feb 13, 2016

RJLiberator

https://www.wolframalpha.com/input/?i=x^2+6x+6

https://www5b.wolframalpha.com/Calculate/MSP/MSP3831i8h4fi8e87f9e4e00000i8261g2b6d6gh03?MSPStoreType=image/gif&s=47 [Broken]

Is the solution then.

Okay, so you convinced me that since b^2-4ac >= 0 then for all b that satisfy this we have reducibility. (I should have known this, it's been a longe week)

With that being stated, there is something more here:
What has b^2-4ac have to do with the roots?

That is the interesting question.
Definition of reducible is if we let f(x) exist in R(x) then f(x) is reducible if there exists g(x) and h(x) in R(x) such that f(x) = g(x)*h(x) and deg(g(x)) < deg(f(x)), deg(h(x)) < deg(f(x))

And so, there clearly exists a g(x) and h(x) by the b^2-4ac relation.

Last edited by a moderator: May 7, 2017
6. Feb 13, 2016

Staff: Mentor

The two roots of $ax^2+bx+c=0$ are $\frac{-b±\sqrt{b^2-4ac}}{2a}$.
So $b^2-4ac=0$ determines exactly one (double counted) root, $b^2-4ac<0$ gives complex roots and $b^2-4ac>0$ gives two real roots. Here we have $b^2-4ac=k^2-4k=k(k-4)≥0$ which is true if both factors are positive or both factors are negative.

7. Feb 13, 2016

RJLiberator

I agree, mate.

And by the definition of reducibility, we see that the roots existing means that the function is reducible for those roots.

Thank you kindly for your help on this problem.