# What ring is isomorphic to factor-ring?

$R=\mathbb{Z}[x]$ - polynomials with integer coefficients

$I = (x^2+x-1)\mathbb{Z}[x]$

In this case we have that classes of factor-ring $R/I$ represented in the next form: $K_{a,b}=ax+b$.

What ring is isomorphic to factor-ring $R/I$? I have suggestion that it will be $\mathbb{R}$, but I'm not sure and can't prove this.

• This will not be $ℝ$ – there are only countably many polynomials in $R = ℤ[X]$, so there are only countably many elements in $R/I$. Dec 11, 2014 at 20:06

Hint:

Put $\;\phi:=\frac{1+\sqrt5}2\;$ , and define

$$f:\;\Bbb Z[x]\to\Bbb Z[\phi]\;,\;\;f(p(x)):=p(\phi)$$

Show the above is an epimorphism of rings. What's $\;\ker f\;$ ? How in the world did I come up with that particular $\;\phi\;$ ?!

• $\;\phi:=\frac{1+\sqrt5}2\;$ because one root of $x^2+x-1$ is $x_0=\frac{-1-\sqrt5}2\;$ Dec 11, 2014 at 20:22
• @Nikita Yes indeed. Dec 11, 2014 at 20:30
• Since $K_{a,b}=ax+b$ and $x_0=\frac{-1-\sqrt5}2\;$. We have $a(\frac{1+\sqrt5}2)+b = b+\frac{a}{2}+\frac{a\sqrt5}{2}$. What next? Am I thinking in the right way or not? Dec 11, 2014 at 22:31
• The elements in $\;\Bbb Z[\phi]\;$ are of the form $\;f(\phi)\;,\;\;f(x)\in\Bbb Z[x]\;$ . You may want to represent these elements as $\;ax +b+I\;,\;\;I:=\langle x^2+x-1\rangle\;,\;\;a,b\in\Bbb Z\;$ , since in the quotient ring every coset has as representative a linear polynomial. I don't know what else to tell you... Dec 12, 2014 at 5:12
• As a result i got that $R/I$ is isomorphic to $\mathbb{Q}(\sqrt5) = \left\{a+b\sqrt5\middle\lvert a,b\in\mathbb{Q}\right\}$ Dec 12, 2014 at 12:08