1
$\begingroup$

If $a(x) + b(x) = x^6-1$ and $\gcd(a(x),b(x))=x+1$ then find a pair of polynomials of $a(x)$,$b(x)$.

Prove or disprove, if there exists more than 1 more distinct values of the polynomials.

  • 0
    I've removed [tag:algebra] tag, since we don't use algebra tag anymore, see [meta](http://meta.math.stackexchange.com/questions/473/the-use-of-the-algebra-tag/3081#3081) for details.2012-10-19

3 Answers 3

2

There is too much freedom. Let $a(x)=x+1$ and $b(x)=(x^6-1)-(x+1)$. Or else use $a(x)=k(x+1)$, $b(x)=x^6-1-k(x+1)$, where $k$ is any non-zero integer.

  • 0
    Thank you.I don't what i was thinking :p2012-10-16
1

Since $\gcd\{a(x),b(x)\}=x+1$, there are polynomials $p(x)$ and $q(x)$ such that $a(x)=(x+1)p(x)$ and $b(x)=(x+1)q(x)$.

$x^6-1=(x+1)\left(x^5-x^4+x^3-x^2+x-1\right)\;,$

so

$p(x)+q(x)=x^5-x^4+x^3-x^2+x-1\;.\tag{1}$

You can easily find several pairs of relatively prime polynomials $p(x)$ and $q(x)$ that satisfy $(1)$, and they give you pairs $a(x),b(x)$.

  • 0
    You just have to make sure that $\gcd(p,q)=1$. For example, let $p(x)=x^k$.2012-10-16
0

Hint $\ \ $ If $\rm\:a\!+\!b = c\:(x\!+\!1)\:$ then

$\rm\:x\!+\!1 = (a,b) = (a,c\,(x\!+\!1)\!-\!a) = (a,c\,(x\!+\!1))\iff a = d(x\!+\!1),\,\ (d,c) = 1$

Thus for $\rm\:c(x\!+\!1) = x^6\!-\!1\:$ it's true $\rm\iff a = d(x\!+\!1),\:$ for $\rm\,d\,$ coprime to $\rm\:c = (x^6\!-\!1)/(x+1),\:$ or, equivalently, $\rm\,d\,$ coprime to $\rm\:x\!-\!1,\ x^2\pm x + 1$