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$$\frac{6 n \left(\frac{2 n^{2}}{\left(n + 1\right)^{2}} - \frac{3 n}{n + 1} + 1\right)}{\left(n + 1\right)^{3}} = 0$$
Solve this equationThe roots of this equation
$$n_{1} = 0$$
$$n_{2} = 1$$
You also need to calculate the limits of y '' for arguments seeking to indeterminate points of a function:
Points where there is an indetermination:
$$n_{1} = -1$$
$$\lim_{n \to -1^-}\left(\frac{6 n \left(\frac{2 n^{2}}{\left(n + 1\right)^{2}} - \frac{3 n}{n + 1} + 1\right)}{\left(n + 1\right)^{3}}\right) = \infty$$
$$\lim_{n \to -1^+}\left(\frac{6 n \left(\frac{2 n^{2}}{\left(n + 1\right)^{2}} - \frac{3 n}{n + 1} + 1\right)}{\left(n + 1\right)^{3}}\right) = -\infty$$
- the limits are not equal, so
$$n_{1} = -1$$
- is an inflection point
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
$$\left[0, 1\right]$$
Convex at the intervals
$$\left(-\infty, 0\right] \cup \left[1, \infty\right)$$