Irreducibility of $f_p(x^{p^{c-1}})$ when given $f_n(x)=x^{n-1} +x^{n-2}+.....+x+1$ where $n$ is positive integer.
If $ f _p(x)=1+x+x^2+ \cdots +x^{p-1} $ where $p$ is prime number, $e$ is any positive integer.