Euler witness
= Euler witness
For an odd integer $N$ and a unit $b$ modulo $N$, the base $b$ is an Euler witness when
$$
b^{(N-1)/2}\not\equiv\left(\frac bN\right)\pmod N.
$$
It therefore certifies that $N$ is composite.
= Euler witness
For an odd integer $N$ and a unit $b$ modulo $N$, the base $b$ is an Euler witness when
$$
b^{(N-1)/2}\not\equiv\left(\frac bN\right)\pmod N.
$$
It therefore certifies that $N$ is composite.