galois.egcd(a: int, b: int) tuple[int, int, int]
galois.egcd(a: Poly, b: Poly)

Finds the multiplicands of $$a$$ and $$b$$ such that $$a s + b t = \mathrm{gcd}(a, b)$$.

Parameters:
a: int
a: Poly

The first integer or polynomial argument.

b: int
b: Poly

The second integer or polynomial argument.

Returns:

• Greatest common divisor of $$a$$ and $$b$$.

• The multiplicand $$s$$ of $$a$$.

• The multiplicand $$t$$ of $$b$$.

Notes

This function implements the Extended Euclidean Algorithm.

References

Examples

Compute the extended GCD of two integers.

In [1]: a, b = 12, 16

In [2]: gcd, s, t = galois.egcd(a, b)

In [3]: gcd, s, t
Out[3]: (4, -1, 1)

In [4]: a*s + b*t == gcd
Out[4]: True


Generate irreducible polynomials over $$\mathrm{GF}(7)$$.

In [5]: GF = galois.GF(7)

In [6]: f1 = galois.irreducible_poly(7, 1); f1
Out[6]: Poly(x, GF(7))

In [7]: f2 = galois.irreducible_poly(7, 2); f2
Out[7]: Poly(x^2 + 1, GF(7))

In [8]: f3 = galois.irreducible_poly(7, 3); f3
Out[8]: Poly(x^3 + 2, GF(7))


Compute the extended GCD of $$f_1(x)^2 f_2(x)$$ and $$f_1(x) f_3(x)$$.

In [9]: a = f1**2 * f2

In [10]: b = f1 * f3

In [11]: gcd, s, t = galois.egcd(a, b)

In [12]: gcd, s, t
Out[12]: (Poly(x, GF(7)), Poly(2x^2 + 4x + 1, GF(7)), Poly(5x^2 + 3x + 4, GF(7)))

In [13]: a*s + b*t == gcd
Out[13]: True