# Poulet number

## Definition

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

.

In other words,  divides . Equivalently,  is a Fermat pseudoprime modulo .

## Occurrence

### Initial examples

The first few Poulet numbers are .

These include, for instance:

### 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  where  itself is a Poulet number OR  where  is prime and  isn't Use that  divides  as a polynomial with integer coefficients.
Composite Fermat number implies Poulet number  where  is not prime Use that  divides  as a polynomial with integer coefficients, and also that  divides  because .
Infinitude of Poulet numbers Use the Mersenne number iteratively, after having found at least one Poulet number.