- galois.Poly.is_irreducible() bool
Determines whether the polynomial \(f(x)\) over \(\mathrm{GF}(p^m)\) is irreducible.
Why is this a method and not a property?
This is a method to indicate it is a computationally expensive task.
- Returns:¶
True
if the polynomial is irreducible.
See also
Notes¶
A polynomial \(f(x) \in \mathrm{GF}(p^m)[x]\) is reducible over \(\mathrm{GF}(p^m)\) if it can be represented as \(f(x) = g(x) h(x)\) for some \(g(x), h(x) \in \mathrm{GF}(p^m)[x]\) of strictly lower degree. If \(f(x)\) is not reducible, it is said to be irreducible. Since Galois fields are not algebraically closed, such irreducible polynomials exist.
This function implements Rabin’s irreducibility test. It says a degree-\(m\) polynomial \(f(x)\) over \(\mathrm{GF}(q)\) for prime power \(q\) is irreducible if and only if \(f(x) \mid (x^{q^m} - x)\) and \(\textrm{gcd}(f(x),\ x^{q^{m_i}} - x) = 1\) for \(1 \le i \le k\), where \(m_i = m/p_i\) for the \(k\) prime divisors \(p_i\) of \(m\).
References¶
Rabin, M. Probabilistic algorithms in finite fields. SIAM Journal on Computing (1980), 273-280. https://apps.dtic.mil/sti/pdfs/ADA078416.pdf
Gao, S. and Panarino, D. Tests and constructions of irreducible polynomials over finite fields. https://www.math.clemson.edu/~sgao/papers/GP97a.pdf
Section 4.5.1 from https://cacr.uwaterloo.ca/hac/about/chap4.pdf
https://en.wikipedia.org/wiki/Factorization_of_polynomials_over_finite_fields
Examples¶
# Conway polynomials are always irreducible (and primitive) In [1]: f = galois.conway_poly(2, 5); f Out[1]: Poly(x^5 + x^2 + 1, GF(2)) # f(x) has no roots in GF(2), a necessary but not sufficient condition of being irreducible In [2]: f.roots() Out[2]: GF([], order=2) In [3]: f.is_irreducible() Out[3]: True
In [4]: g = galois.irreducible_poly(2**4, 2, method="random"); g Out[4]: Poly(x^2 + 13x + 10, GF(2^4)) In [5]: h = galois.irreducible_poly(2**4, 3, method="random"); h Out[5]: Poly(x^3 + 7x^2 + 5x + 14, GF(2^4)) In [6]: f = g * h; f Out[6]: Poly(x^5 + 10x^4 + 10x^3 + x^2 + 14x + 6, GF(2^4)) In [7]: f.is_irreducible() Out[7]: False