Determine whether $f$ is a function from $Z$ to $R.$ Enter $"$Y$"$ for yes and $"$N$"$ for no$.$

$f(n)=\frac{1}{n^2-9}$

$f(n)=\frac{1}{n^2+6}$

$f(n)=\sqrt[\phantom{2}]{n^2+7}$

$f(n)=+-n$