# polynomial

• Oct 12th 2009, 04:52 AM
Sampras
polynomial
Let \$\displaystyle J \$ be the set of all polynomials with zero constant term in \$\displaystyle \mathbb{Z}[x] \$.

(a) Show that \$\displaystyle J \$ is the principal ideal \$\displaystyle (x) \$ in \$\displaystyle \mathbb{Z}[x] \$.

(b) Show that \$\displaystyle \mathbb{Z}[x]/J \$ consists of an infinite number of distinct cosets.

So for (a) suppose it is not? For (b) you would also use contradiction?
• Oct 12th 2009, 09:15 AM
eeyore
I would think about it a little more directly. First, J is an ideal because any polynomial multiplied by an element of J will give you an element of J. It is principal because it is generated by x.

For the second part, I can't seem to give a rigourous explanation that I am happy with. Try to write a few of the cosets.
\$\displaystyle \lbrace 1 + x, 2 + x, 3 + x,\ldots\rbrace\$
\$\displaystyle \lbrace 1 + x^2, 2 + x^2, 3 + x^2,\ldots, x+x^2, 2x+x^2, 3x+x^2 \ldots \rbrace\$

They are all different and there is an infinite number of them because you can just keep raising x to a higher power. I hope I didn't confuse you with that. I'll think about it some more and try to come up with a better explanation.