# Equivalence of Definitions of Euler Lucky Number

## Theorem

The following definitions of the concept of Euler Lucky Number are equivalent:

### Definition 1

The Euler lucky numbers are the prime numbers $p$ such that:

$n^2 + n + p$

is prime for $0 \le n < p - 1$.

### Definition 2

The Euler lucky numbers are the prime numbers $p$ such that:

$n^2 - n + p$

is prime for $1 \le n < p$.

## Proof

Let $p$ be an Euler lucky number.

Let $f_p: \Z \to \Z$ be the mapping defined as:

$\forall n \in \Z: f_p \left({n}\right) = n^2 + n + p$

Let $m = n - 1$.

Then:

 $\displaystyle f_p \left({m}\right)$ $=$ $\displaystyle f_p \left({n - 1}\right)$ $\displaystyle$ $=$ $\displaystyle \left({n - 1}\right)^2 + \left({n - 1}\right) + p$ $\displaystyle$ $=$ $\displaystyle n^2 - 2 n + 1 + n - 1 + p$ $\displaystyle$ $=$ $\displaystyle n^2 - n + p$

We have that $f_p \left({n}\right)$ is prime for $0 \le n < p - 1$.

Thus $f_p \left({m}\right)$ is prime for $0 \le \left({n - 1}\right) < p - 1$.

and so $f_p \left({m}\right)$ is prime for $1 \le n < p$.

$\blacksquare$