# factorization of primitive polynomial

As an application of the parent entry (http://planetmath.org/EliminationOfUnknown) we take the factorization of a primitive polynomial of $\mathbb{Z}[x]$ into primitive (http://planetmath.org/PrimitivePolynomial) prime factors.  We shall see that the procedure may be done by performing a finite number of tests.

Let

 $a(x)\;=:\;a_{n}x^{n}\!+\!a_{n-1}x^{n-1}\!+\ldots+\!a_{0}$

be a primitive polynomial in $\mathbb{Z}[x]$.

By the rational root theorem and the factor theorem, one finds all first-degree prime factors $x\!-\!a$ and thus all primitive prime factors of the polynomial $a(x)$.

If $a(x)$ has a primitive quadratic factor, then it has also a factor

 $\displaystyle x^{2}\!+\!px\!+\!q$ (1)

where $p$ and $q$ are rationals (and conversely).  For settling the existence of such a factor we treat $p$ and $q$ as unknowns and perform the long division

 $a(x):(x^{2}\!+\!px\!+\!q).$

It gives finally the remainder  $b(p,\,q)\,x+c(p,\,q)$ where $b(p,\,q)$ and $c(p,\,q)$ belong to $\mathbb{Z}[p,\,q]$.  According to the parent entry (http://planetmath.org/EliminationOfUnknown) we bring the system

 $\displaystyle\begin{cases}b(p,\,q)\;=\;0\\ c(p,\,q)\;=\;0\end{cases}$

to the form

 $\displaystyle\begin{cases}\bar{b}(q)\;=\;0\\ \bar{c}(p,\,q)\;=\;0\end{cases}$

and then can determine the possible rational solutions  $(p,\,q)$  of the system via a finite number of tests.  Hence we find the possible quadratic factors (1) having rational coefficients.  Such a factor is converted into a primitive one when it is multiplied by the gcd of the denominators of $p$ and $q$.

Determining a possible cubic factor $x^{3}\!+\!px^{2}\!+\!qx\!+\!r$ with rational coefficients requires examination of a remainder of the form

 $b(p,\,q,\,r)\,x^{2}+c(p,\,q,\,r)\,x+d(p,\,q,\,r).$

In the needed system

 $\displaystyle\begin{cases}b(p,\,q,\,r)\;=\;0\\ c(p,\,q,\,r)\;=\;0\\ d(p,\,q,\,r)\;=\;0\end{cases}$

we have to perform two eliminations.  Then we can act as above and find a primitive cubic factor of $a(x)$.  Similarly also the primitive factors of higher degree.  All in all, one needs only look for factors of degree $\leq\frac{n}{2}$.

## References

• 1 K. Väisälä: Lukuteorian ja korkeamman algebran alkeet.  Tiedekirjasto No. 17.   Kustannusosakeyhtiö Otava, Helsinki (1950).
Title factorization of primitive polynomial FactorizationOfPrimitivePolynomial 2013-03-22 19:20:30 2013-03-22 19:20:30 pahio (2872) pahio (2872) 4 pahio (2872) Algorithm msc 12D99 msc 26C05 primitive factors of primitive polynomial EliminationOfUnknown