Let's find the inflection points, we'll need to solve the equation for this
$$\frac{d^{2}}{d n^{2}} f{\left(n \right)} = 0$$
(the second derivative equals zero),
the roots of this equation will be the inflection points for the specified function graph:
$$\frac{d^{2}}{d n^{2}} f{\left(n \right)} = $$
the second derivative$$\frac{\frac{8 n^{2} \left(n + 1\right)}{n^{2} + 3} - 6 n + \frac{\left(n + 1\right) \left(\frac{2 n \left(n + 1\right)}{n^{2} + 3} - 1\right)^{2}}{\left(n^{2} + 3\right) \left(- \frac{\left(n + 1\right)^{2}}{\left(n^{2} + 3\right)^{2}} + 1\right)} - 2}{\left(n^{2} + 3\right)^{2} \sqrt{- \frac{\left(n + 1\right)^{2}}{\left(n^{2} + 3\right)^{2}} + 1}} = 0$$
Solve this equationThe roots of this equation
$$n_{1} = -4.74897277059013$$
$$n_{2} = -0.257681423805139$$
$$n_{3} = 2.00380515201852$$
Сonvexity and concavity intervals:Let’s find the intervals where the function is convex or concave, for this look at the behaviour of the function at the inflection points:
Concave at the intervals
$$\left[-4.74897277059013, -0.257681423805139\right] \cup \left[2.00380515201852, \infty\right)$$
Convex at the intervals
$$\left(-\infty, -4.74897277059013\right] \cup \left[-0.257681423805139, 2.00380515201852\right]$$