# Challenge problem from class

1. May 6, 2004

### qaz

show that the polynomial $g(x)=(x^2 -5) (x^2-41)(x^2-205)$ has a solution modulo any integer $n$∈ℕ.

2. May 6, 2004

### matt grime

you mean root; if there is a solution it should come from considering quadratic residues, i imagine.

3. May 7, 2004

### qaz

ok, but i am stil stuck i dont know where to go from here...we dont have a good book for this class and it wasnt explained well at all.

4. May 7, 2004

### Hurkyl

Staff Emeritus
Have you done anything? (whether successful or not)

Sometimes starting small helps. What has to be true for some number m to be a root of g, mod n? What has to be true if no such m exists?

5. May 8, 2004

### matt grime

Have you also noticed something important about 205?

What do you konw about Legendre's Symbol? I'm sure there are lots of useful resources out there, try Wolfram.

6. May 9, 2004

### qaz

ok, i know that 205 can be written as the following: (205/n)=(5/n)(41/n), which reduces to =(-1)(-1)=1. so there are either 2 cases for this problem, (5/n)=1 or (41/n)=1.

7. May 10, 2004

### matt grime

that is possibly correct in spirit, but needs better explanation: at least one of 5,41 or 41*205 must be a quadratic residue mod n for each n because...