Let's find the inflection points, we'll need to solve the equation for this
$$\frac{d^{2}}{d x^{2}} f{\left(x \right)} = 0$$
(the second derivative equals zero),
the roots of this equation will be the inflection points for the specified function graph:
$$\frac{d^{2}}{d x^{2}} f{\left(x \right)} = $$
the second derivative$$\frac{810 \left(- 9 x \operatorname{atan}{\left(9 x \right)} + 2\right) \operatorname{atan}^{3}{\left(9 x \right)}}{\left(81 x^{2} + 1\right)^{2}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 40810.6197109511$$
$$x_{2} = 20468.3034200191$$
$$x_{3} = 29791.8335213252$$
$$x_{4} = -27965.4086855565$$
$$x_{5} = 39963.0194213379$$
$$x_{6} = -28813.0049161886$$
$$x_{7} = 33182.2243847643$$
$$x_{8} = 23858.6674225697$$
$$x_{9} = -33050.9922475461$$
$$x_{10} = -38984.1869079529$$
$$x_{11} = -21184.662014324$$
$$x_{12} = -39831.7869983294$$
$$x_{13} = 32334.6261545405$$
$$x_{14} = -27117.8129572642$$
$$x_{15} = 38267.819367964$$
$$x_{16} = 34877.4217234979$$
$$x_{17} = -38136.5870023501$$
$$x_{18} = -25422.6232094936$$
$$x_{19} = -40679.3872619006$$
$$x_{20} = -17794.3137502869$$
$$x_{21} = -32203.3940662699$$
$$x_{22} = 22163.4833872945$$
$$x_{23} = 39115.4193032007$$
$$x_{24} = -33898.5907325275$$
$$x_{25} = -31355.796213409$$
$$x_{26} = -22032.2523985593$$
$$x_{27} = -34746.1894989223$$
$$x_{28} = -30508.1987164287$$
$$x_{29} = 37420.2196280905$$
$$x_{30} = 34029.8229150639$$
$$x_{31} = 35725.0207902128$$
$$x_{32} = 23011.0749528733$$
$$x_{33} = 30639.4306942676$$
$$x_{34} = 42505.8207634747$$
$$x_{35} = 28944.2367635107$$
$$x_{36} = 31487.0282487046$$
$$x_{37} = -26270.2177801261$$
$$x_{38} = 26401.449382206$$
$$x_{39} = 25553.8547130074$$
$$x_{40} = -37288.987294156$$
$$x_{41} = -20337.0727905792$$
$$x_{42} = 27249.0446488013$$
$$x_{43} = 17925.5436357075$$
$$x_{44} = -23727.4361488676$$
$$x_{45} = 36572.6200972009$$
$$x_{46} = 21315.8928342054$$
$$x_{47} = 24706.260702945$$
$$x_{48} = -35593.7885265682$$
$$x_{49} = -18641.8984612561$$
$$x_{50} = -24575.0293083862$$
$$x_{51} = 28096.6404585308$$
$$x_{52} = -36441.3877971843$$
$$x_{53} = -41526.987688035$$
$$x_{54} = -19489.4848795393$$
$$x_{55} = 41658.2201615479$$
$$x_{56} = -42374.5882669537$$
$$x_{57} = -29660.6016059448$$
$$x_{58} = 0$$
$$x_{59} = -22879.8438137284$$
$$x_{60} = 18773.1286287822$$
$$x_{61} = 19620.7152931068$$
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
$$\left[0, \infty\right)$$
Convex at the intervals
$$\left(-\infty, 0\right]$$