Poulet number

Definition

A Poulet number or Sarrus number is an odd composite number $n$ such that:

$2^{n-1}\equiv 1modn$.

In other words, $n$ divides $2^{n-1}-1$. Equivalently, $n$ is a Fermat pseudoprime modulo $2$.

Occurrence

Initial examples

The first few Poulet numbers are $341,561,645,1105,1387,1729,1905,2047$.

These include, for instance:

• The Mersenne number $M_{11}=2047$, which is a Poulet number on account of the fact that Mersenne number for prime or Poulet implies prime or Poulet.
• Three Carmichael numbers -- these are numbers that are pseudoprime to every relatively prime base. These are $561,1105,1729$. $1729$ is also known as the Hardy-Ramanujan number, and is the smallest number expressible as the sum of two cubes in two distinct ways.

Infinitude

Further information: Infinitude of Poulet numbers

There are infinitely many Poulet numbers. This can be proved in many ways. For instance, Mersenne number for prime or Poulet implies prime or Poulet. This shows that if we find one Poulet number, we can iterate the operation of taking the Mersenne number and obtain infinitely many Poulet numbers.

Facts

Statement	Kind of numbers it says are Poulet numbers	Proof idea
Mersenne number for prime or Poulet implies prime or Poulet	$M_n=2^n-1$ where $n$ itself is a Poulet number OR $M_n$ where $n$ is prime and $M_n$ isn't	Use that $x-1$ divides $x^m-1$ as a polynomial with integer coefficients.
Composite Fermat number implies Poulet number	$F_n=2^{2^n}+1$ where $F_n$ is not prime	Use that $x-1$ divides $x^m-1$ as a polynomial with integer coefficients, and also that $2^{n+1}$ divides $2^{2^n}$ because $n+1\le 2^n$.
Infinitude of Poulet numbers		Use the Mersenne number iteratively, after having found at least one Poulet number.

Relation with other properties

Stronger properties

• Absolute pseudoprime (at least, for odd numbers).