Wednesday, 2 September 2009

Finite fields, part 2

Okay, so last time, we saw how to extend Q, the field of rational numbers, by adjoining a new element which is the zero of a polynomial in Q[X]. For example, we can adjoin sqrt 2, a zero of X^2-2, to obtain the field Q(sqrt2).

The time before that, we saw that for each prime p, there is a finite field Fp, consisting of the set {0,1,...,p-1}, with the arithmetic operations carried out modulo p.

This week we're going to bring these two ideas together, and look at algebraic extensions of Fp.

Algebraic extensions of Fp work just the same way as algebraic extensions of Q. First, find an irreducible polynomial over the base field K - that means, a polynomial f(x) in K[x], which can't be expressed as f(x) = g(x) * h(x), for g(x), h(x) in K[x] of lower degree. For example, whenever d is an element of K without a square root in K, then x^2-d is irreducible. Then, form the quotient ring K[x]/ - that means, work in K[x], but use the division with remainder algorithm to set f=0, which has the effect of making x act like a zero of f. For f(x)=x^2-d, this means x ends up acting like sqrt d.

For example, in F3, and also in F5, there is no square root of 2. So we can form the fields F3(sqrt2) or F5(sqrt2). We need to be a bit careful about terminology. 2 in F3, 2 in F5, and 2 in Q, are all different things - consequently sqrt2 in F3, in F5, and in Q are all different things. We mustn't make the mistake of thinking they're the same thing.

We saw last time that Q(sqrt2) consists of { a + b sqrt2 | a, b in Q }. Similarly, F5(sqrt2) consists of { a + b sqrt2 | a, b in F5 }. In particular, it follows that F5(sqrt2) has 25 elements. In general, a simple algebraic extension of Fp will have p^n elements, where n is the degree of the irreducible polynomial.

Over Q, we were able to form the fields Q(sqrt2), Q(sqrt3), Q(i), and so on. Somewhat surprisingly, over Fp, it turns out that any algebraic extension of degree n is isomorphic to any other. For example, F5(sqrt2) is isomorphic to F5(sqrt3). For this reason, we typically just talk about F25 (meaning, the field with 25 elements) - or in general, Fq, where q = p^n is a prime power.

However, in order to do arithmetic in F25 or in Fq, we clearly need to have some particular irreducible polynomial in mind. Otherwise, we won't know whether to reduce x^2 to 2, or to 3, for example. So for each p, n, we need to agree on an irreducible polynomial of degree n over Fp. There doesn't appear to be any natural way to choose, so an artificial way to choose has been devised, called the Conway polynomials.

So, using a little phantom type trickery as before, here are the first few finite fields of prime power order, as defined in the HaskellForMaths library:

data ConwayF4
instance PolynomialAsType F2 ConwayF4 where pvalue _ = convert $ x^2+x+1
type F4 = ExtensionField F2 ConwayF4
f4 = map Ext (polys 2 f2) :: [F4]
a4 = embed x :: F4

data ConwayF8
instance PolynomialAsType F2 ConwayF8 where pvalue _ = convert $ x^3+x+1
type F8 = ExtensionField F2 ConwayF8
f8 = map Ext (polys 3 f2) :: [F8]
a8 = embed x :: F8

data ConwayF9
instance PolynomialAsType F3 ConwayF9 where pvalue _ = convert $ x^2+2*x+2
type F9 = ExtensionField F3 ConwayF9
f9 = map Ext (polys 2 f3) :: [F9]
a9 = embed x :: F9

data ConwayF16
instance PolynomialAsType F2 ConwayF16 where pvalue _ = convert $ x^4+x+1
type F16 = ExtensionField F2 ConwayF16
f16 = map Ext (polys 4 f2) :: [F16]
a16 = embed x :: F16

data ConwayF25
instance PolynomialAsType F5 ConwayF25 where pvalue _ = convert $ x^2+4*x+2
type F25 = ExtensionField F5 ConwayF25
f25 = map Ext (polys 2 f5) :: [F25]
a25 = embed x :: F25

As we did with the fields Fp, we provide the functions f4, f8, f9, etc, which just return a list of the elements of the corresponding fields. a4, a8, a9 etc return the element that has been adjoined to the underlying field to make the extension field.

For example:

> f8
[0,a^2,a,a+a^2,1,1+a^2,1+a,1+a+a^2] :: [F8]
> a8 ^ 3
1+a :: F8

We can partially verify the claim that F25 = F5(sqrt2) = F5(sqrt3), by exhibiting square roots of 2 and 3 in F25:

> (2+a25)^2
2 :: F25
> (1+3*a25)^2
3 :: F25

Last time, I mentioned that extension fields can have automorphisms, without really spelling out what that means. Well, if L is an extension of K, then f is an automorphism of the extension if for a,b in L, c in K, f(a+b) = f(a)+f(b), f(a*b) = f(a)*f(b), etc, and f(c*a)= c*f(a). In other words, f(L) looks the same as L, viewed from K.

The finite fields Fq (q = p^n) have an automorphism called the Frobenius automorphism, defined by f(x)=x^p. For example:

> f9
> map frobeniusAut f9

Finally, a word about efficiency. The HaskellForMaths implementation of fields Fq is very natural from a mathematical point of view, but it isn't very efficient.

Firstly, we are working with polynomials with coefficients in Fp. To multiply two degree n polynomials together, we have to do O(n^2) additions and multiplications of coefficients. If the coefficients are in Fp, each one of these involves a (`mod` p) operation. It would be better to do the polynomial arithmetic over the integers, and only do the `mod` p at the end. We would then only have to do O(n) (`mod` p) operations.

Secondly, we could consider implementing the fields Fq using Zech logarithms instead.

Anyway, it doesn't matter too much for the moment, as I only want to use the finite fields to construct combinatorial structures. But sometime I might try to write a more efficient implementation.

Next time: finite geometries.


  1. Hi,
    thanks for this very interesting library.
    I think something's missing in Math/Algebra/Field/Extension :

    > +set +t
    > 1.4 :: Q
    it :: Q
    > 1.4 :: QSqrt3
    *** Exception: Math/Algebra/Field/Extension.hs:116:0: No instance nor default method for class operation GHC.Real.fromRational

  2. Hmm, thanks for pointing this out. When I added a definition for fromRational, I discovered a subtle bug in the code:
    > 1/5 :: QSqrt3
    I now have fixes for both problems, which will be in the next release.