# Fermat Quotient of 2 wrt p is Square iff p is 3 or 7

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## Theorem

Let $p$ be a prime number.

The Fermat quotient of $2$ with respect to $p$:

- $\map {q_p} 2 = \dfrac {2^{p - 1} - 1} p$

is a square if and only if $p = 3$ or $p = 7$.

## Proof

When $p = 3$:

- $\map {q_3} 2 = \dfrac {2^{3 - 1} - 1} 3 = 1$

which is square.

When $p = 7$:

- $\map {q_7} 2 = \dfrac {2^{7 - 1} - 1} 7 = \dfrac {63} 7 = 9$

which is square.

## Sources

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $7$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $7$