Carmichael number
In number theory a Carmichael number is a composite positive integer [math]\displaystyle{ n }[/math], which satisfies the congruence [math]\displaystyle{ b^{n-1}\equiv 1\pmod{n} }[/math] for all integers [math]\displaystyle{ b }[/math] which are relatively prime to [math]\displaystyle{ n }[/math]. Being relatively prime means that they do not have common divisors, other than 1. Such numbers are named after Robert Carmichael.
All prime numbers [math]\displaystyle{ p }[/math] satisfy [math]\displaystyle{ b^{p-1}\equiv 1\pmod{p} }[/math] for all integers [math]\displaystyle{ b }[/math] which are relatively prime to [math]\displaystyle{ p }[/math]. This has been proven by the famous mathematician Pierre de Fermat. In most cases if a number [math]\displaystyle{ n }[/math] is composite, it does not satisfy this congruence equation. So, there exist not so many Carmichael numbers. We can say that Carmichael numbers are composite numbers that behave a little bit like they would be a prime number.
Carmichael Number Media
Václav Šimerka listed the first seven Carmichael numbers
