Zsigmondy's theorem

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In number theory, Zsigmondy's theorem, named after Karl Zsigmondy, states that if a > b > 0 are coprime integers, then for any natural number n > 1 there is a prime number p (called a primitive prime divisor) that divides anbn and does not divide akbk for any positive integer k < n, with the following exceptions:

  • a = 2, b = 1, and n = 6; or
  • a + b is a power of two, and n = 2.

This generalizes Bang's theorem, which states that if n > 1 and n is not equal to 6, then 2n − 1 has a prime divisor not dividing any 2k − 1 with k < n.

Similarly, an + bn has at least one primitive prime divisor with the exception 23 + 13 = 9.

Zsigmondy's theorem is often useful, especially in group theory, where it is used to prove that various groups have distinct orders except when they are known to be the same.


The theorem was discovered by Zsigmondy working in Vienna from 1894 until 1925.


Let (a_n)_{n\ge1} be a sequence of nonzero integers. The Zsigmondy set associated to the sequence is the set

\mathcal{Z}(a_n) = \{ n \ge 1 : a_n \text{ has no primitive prime divisors} \}.

i.e., the set of indices n such that every prime dividing a_n also divides some a_m for some m < n. Thus Zsigmondy's theorem implies that \mathcal{Z}(a^n-b^n)\subset\{1,2,6\}, and Carmichael's theorem says that the Zsigmondy set of the Fibonacci sequence is \{1,2,6,12\}. In 2001 Bilu, Hanrot, and Voutier[1] proved that in general, if (a_n)_{n\ge1} is a Lucas sequence or a Lehmer sequence, then \mathcal{Z}(a_n) \subseteq \{ 1 \le n \le 30 \}. Lucas and Lehmer sequences are examples of divisibility sequences.

It is also known that if (W_n)_{n\ge1} is an elliptic divisibility sequence, then its Zsigmondy set \mathcal{Z}(W_n) is finite.[2] However, the result is ineffective in the sense that the proof does give an explicit upper bound for the largest element in \mathcal{Z}(W_n), although it is possible to give an effective upper bound for the number of elements in \mathcal{Z}(W_n).[3]

See also[edit]


  1. ^ Y. Bilu, G. Hanrot, P.M. Voutier, Existence of primitive divisors of Lucas and Lehmer numbers, J. Reine Angew. Math. 539 (2001), 75-122
  2. ^ J.H. Silverman, Wieferich's criterion and the abc-conjecture, J. Number Theory 30 (1988), 226-237
  3. ^ P. Ingram, J.H. Silverman, Uniform estimates for primitive divisors in elliptic divisibility sequences, Number theory, Analysis and Geometry, Springer-Verlag, 2010, 233-263.

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