  We give a very short and simple proof of Zykov's generalization of
Tur\'{a}n's theorem, which implies that the number of maximum independent sets
of a graph of order $n$ and independence number $\alpha$ with $\alpha<n$ is at
most $\left\lceil\frac{n}{\alpha}\right\rceil^{n\,{\rm mod}\,\alpha}
\left\lfloor\frac{n}{\alpha}\right\rfloor^{\alpha-(n\,{\rm mod}\,\alpha)}$.
Generalizing a result of Zito, we show that the number of maximum independent
sets of a tree of order $n$ and independence number $\alpha$ is at most
$2^{n-\alpha-1}+1$, if $2\alpha=n$, and, $2^{n-\alpha-1}$, if $2\alpha>n$, and
we also characterize the extremal graphs. Finally, we show that the number of
maximum independent sets of a subcubic tree of order $n$ and independence
number $\alpha$ is at most $\left(\frac{1+\sqrt{5}}{2}\right)^{2n-3\alpha+1}$,
and we provide more precise results for extremal values of $\alpha$.

Combinatorics