Is there a simple way to determine if an element is a primitive root modulo n?

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    Primitive Root
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Discussion Overview

The discussion revolves around the methods for determining whether an element is a primitive root modulo a prime number P. Participants explore the complexity of these methods, particularly in relation to the factorization of P-1 and the conditions that must be satisfied for an element to be considered a generator in the multiplicative group of integers mod P.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about a polynomial-time algorithm to determine if an element is a generator in the multiplicative group of integers mod P.
  • Another participant suggests a method that requires the factorization of P-1 into its prime factors and provides a condition involving congruences to verify if an element is a primitive root.
  • A correction is made regarding the exponent in the congruence condition necessary for an element to be a primitive root, emphasizing that it should not be congruent to 1 for any value less than φ(P).
  • A participant acknowledges the difficulty of factorizing P-1 and notes that while checking if g is a primitive root can be done in polynomial time if P-1 is known, the overall problem remains complex.
  • Historical context is provided, referencing Gauss's work on primitive roots and the lack of easy methods to find them, suggesting that the situation has not significantly changed since then.

Areas of Agreement / Disagreement

Participants express differing views on the ease and feasibility of finding primitive roots, with some acknowledging the complexity involved in factorization and the conditions required for an element to qualify as a primitive root. There is no consensus on a straightforward method.

Contextual Notes

The discussion highlights the unresolved nature of factorization algorithms and the dependence on the properties of P-1, which may limit the applicability of proposed methods.

sid_galt
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Is there an easy (by which I mean an algorithm polynomial in size of input) way to know whether in the multiplicative group of integers mod P (P is a prime), whether an element is a generator or not?
 
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Hi, there is one method but it required knowing expansion of the number P-1 on the simple factors.

So, if P-1 = p_{1}^{a_1} \times \cdots \times p_{n}^{a_n} where each p_i is prime and for each i=1...n : \frac{g^{P-1}}{p_i} \equiv 1 (mod P), than g - is primitive root (generator) by modulo P.

Unfortunately, exponential factorization algorithms not known yet.
 
Last edited:
ialbekht: Hi, there is one method

THAT! You are submitting as an EASY method? Anyway you got the exponent wrong. Should be:

i=1...n : g^\frac{P-1}{p_i} \equiv 1 (mod P) Also, to be a primitative root, it would NOT--by definition--be congruent to 1 for any value less than\varphi({P}).
 
Last edited:
robert Ihnot:
Thanks for correcting me.

It's the first method I've remembered. The hardest thing is to factorize P-1, and It's not a problem for a not large P and if you know factorization of P-1 you may check g to be primitive root for polynomial time.

And I did not say that it's EASY way. Unfortunately, there are not much easy solutions in number-theory problems :wink:
 
From Wickipedia: Gauss defines primitive root in Article 57 of the Disquisitiones Arithmeticae (1801), where he credits Euler with coining the term.

Gauss then went ahead and gave the reader a table of primitive roots, so that he was aware that there was no really easy way to find them. It then seems, from the responses we are getting here, that's about where it is today!
 

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