Coset Decision Method
📂Number TheoryCoset Decision Method
Overview
The theorem, which serves as a necessary and sufficient condition for determining if a number is a Carmichael number, can also be usefully applied.
Theorem
Let an odd composite number be n.
n is a Carmichael number satisfying ⟺ p∣n for all primes p such that p2∤n and (p−1)∣(n−1).
Proof
Definition of Carmichael Numbers: A natural number n is called a Carmichael number if it satisfies an≡a(modn) for all 1≤a≤n.
(⟹)
A Carmichael number can be expressed as n=p1⋯pm for different primes p1,⋯,pm, excluding 2, and therefore must satisfy pi2∤n.
Since n is a Carmichael number, it follows that an−1≡1(modn) and for any k,
an−1=n⋅k+1=pi⋅(pink)+1
therefore an−1≡1(modpi).
Properties of Order: For a prime p, if gcd(a,p)=1 and an≡1(modp) then ordp(a)∣n
According to the properties of order, ordpi(a) divides (n−1), and by the Primitive Root Theorem, having at least 1 primitive roots, for some ai,
ordpi(ai)=pi−1
Therefore, for all p1,⋯,pm, (pi−1)∣(n−1) must hold.
(⟸)
Let’s assume n:=p1⋯pm.
For all primes p1,⋯,pm dividing n, since p2∤n, p1,⋯,pm are distinct.
Since (p−1)∣(n−1), for some integer ki,
(n−1)=(pi−1)ki
Now, consider 1≤a≤n and whether it divides or does not divide pi.
- Case 1. pi∣a
Obviously, an≡0≡a(modpi) holds. - Case 2. pi∤a
By Fermat’s Little Theorem,
an==≡≡a(pi−1)ki+1a(pi−1)ki⋅a1ki⋅a(modpi)a(modpi)
Therefore, in any case,
(an−a)≡0(modpi)
which means that (an−a) is divisible by p1,⋯,pm. In other words, (an−a) is divisible by n=p1⋯pm, summarizing we obtain the following.
(an−a)≡0(modn)
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