  The problem of solving explicitly the equation $P_a(X):=X^{q+1}+X+a=0$ over
the finite field $\GF{Q}$, where $Q=p^n$, $q=p^k$ and $p$ is a prime, arises in
many different contexts including finite geometry, the inverse Galois problem
\cite{ACZ2000}, the construction of difference sets with Singer parameters
\cite{DD2004}, determining cross-correlation between $m$-sequences
\cite{DOBBERTIN2006} and to construct error correcting codes
\cite{Bracken2009}, cryptographic APN functions
\cite{BTT2014,Budaghyan-Carlet_2006}, designs \cite{Tang_2019}, as well as to
speed up the index calculus method for computing discrete logarithms on finite
fields \cite{GGGZ2013,GGGZ2013+} and on algebraic curves \cite{M2014}.
  Subsequently, in
\cite{Bluher2004,HK2008,HK2010,BTT2014,Bluher2016,KM2019,CMPZ2019,MS2019,KCM19},
the $\GF{Q}$-zeros of $P_a(X)$ have been studied. In \cite{Bluher2004}, it was
shown that the possible values of the number of
  the zeros that $P_a(X)$ has in $\GF{Q}$ is $0$, $1$, $2$ or $p^{\gcd(n,
k)}+1$.
  Some criteria for the number of the $\GF{Q}$-zeros of $P_a(x)$ were
  found in \cite{HK2008,HK2010,BTT2014,KM2019,MS2019}. However, while the
ultimate goal is to explicit all the $\GF{Q}$-zeros,
  even in the case $p=2$, it was solved only under the condition $\gcd(n, k)=1$
\cite{KM2019}.
  In this article, we discuss this equation without any restriction on $p$ and
$\gcd(n,k)$. In \cite{KCM19}, for the cases of one or two $\GF{Q}$-zeros,
explicit expressions for these rational zeros in terms of $a$ were provided,
but for the case of $p^{\gcd(n, k)}+1$ $\GF{Q}-$ zeros it was remained open to
explicitly compute the zeros. This paper solves the remained problem, thus now
the equation $X^{p^k+1}+X+a=0$ over $\GF{p^n}$ is completely solved for any
prime $p$, any integers $n$ and $k$.
