# Square-free polynomial factorization in finite field

The calculator finds all square factors of polynomial in finite field.

The following calculator finds all square factors of a polynomial in the finite field. The square-free factorization is the first step in the polynomial factor decomposition process. You can find more details just below the calculator.

We already have similar calculator for square free decomposition in the field of rational numbers, but for some edge cases it will not work for a polynomial with coefficients in finite field. A square-free polynomial decomposition algorithm is based on calculation of the greatest common divisor (GCD) of the polynomial and its derivative : gcd(A,A').

The derivative of a non-zero degree polynomial is always not zero in the ring of integers or rational field. But it can become zero in finite field. E.g. the polynomial x^{6}+x^{3}+2 has zero derivative in F_{3} field, since (6 mod 3)x^{5}+(3 mod 3)x^{2} = 0x^{5}+0x^{2} = 0. A polynomial term with a degree that's a multiple of a field order becomes zero in the derivative.

### Square-free decomposition algorithm in finite field

The square-free factorization algorithm for finite field addresses the zero derivative issue, discussed above. The calculator uses the algorithm, described in wikipedia^{1} ( without recursion ):

```
// a(x) - Input polynomial (must be monic)
// p - field order
m⟵1
do
//Greatest common divisor calculation
c(x) ⟵ gcd( a(x), a'(x) )
w(x) ⟵ a(x)/c(x)
i ⟵1
loop while w(x) !=1
y(x) ⟵ gcd(w(x), c(x));
q(x) ⟵ w(x) / y(x)
if q(x)!=1 output ⟵ q(x)^i*m
w(x) = y(x)
c(x) = c(x)/y(x)
i=i+1
end loop
if c(x)!=1
a(x) = c(x)^(1/p)
m ⟵ p*m
end if
loop until c(x)!=1
```

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