Wednesday, 3 September 2008

Complex powers in finite fields

There is no standard way to define this idea, and I doubt there is a useful one.



If I were trying to invent a definition, I'd say the following:



We should definitely have (ab)i=aibi and (ai)i=a1. The first says that taking the i-th power is a homomorphism from the multiplicative group of the field to itself. Since the multiplicative group of a finite field is cyclic, the only homomorphisms from it to itself are of the form amapstoak for some integer k.



Which integers k should we accept? The second property imposes ak2=a1 modulo p. This implies that k2=1mod(p1).



For most primes p, there are no solutions to k2=1mod(p1). In the cases that there are, you could define raising to the i power to means raising to the k power for such a k. For example, when p=11, you could define ai to mean a3, and you would get the two properties above.



I wouldn't do this though. At least in the areas of math where I work, it is considered a bad idea to define an operation on Fp that doesn't extend nicely to the fields Fpj. And there will never be a good definition of raising to the i power in Fp2. In order to get one, you'd need a solution to k2=1modp21. But, for any prime other than 3, p21 is divisible by 3, and k2+1 is never divisible by 3.

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