# If a prime number $p$ satisfies $\gcd(a,p−1) = 1$, then for every integer $b$ the congruence relation $x^a \equiv b \pmod{p}$ has a solution.

If a prime number $p$ satisfies $\gcd(a,p−1) = 1$, then for every integer $b$ the congruence relation $x^a \equiv b \pmod{p}$ has a solution.

Using Discrete Logarithm wrt primitive root $g\pmod p$
$\displaystyle a\cdot$ind$_gx\equiv$ind$_gb\pmod{p-1}$
Using Linear congruence theorem (Proof), as $\displaystyle d=(a,p-1)=1$ always divides ind$_gb,$ the number of solutions will be $d$
• It is not without interest to note that there are cases in which $a|{p-1}$ and $x^a=b$ could have solution for all $b$ excepting $b=0$. Oct 23 '17 at 0:27
The set $S=\{1^a,2^a,\cdots,(p-1)^a,p^a\}$ is contained in $\mathbb F_p$ and can be equal or distinct of it. If $(a,p-1)=1$ then for $b=0$ in $\mathbb F_p$ we have $p^a=0$ and $x^a=0$ has a solution. Let $b\ne0$. If $x^a=b$ has no solution, then $x^a=y^a$ for some distinct elements of the group multiplicative $\mathbb F_p^*$ then $z^a=1$ where $z=x\cdot y^{-1}$. But this implies $a$ divides $p-1$ by FLT, contradiction.