Given number: $$a x$$ It is a series of species $$a_{a} \left(c x - x_{0}\right)^{a d}$$ - power series. The radius of convergence of a power series can be calculated by the formula: $$R^{d} = \frac{x_{0} + \lim_{a \to \infty} \left|{\frac{a_{a}}{a_{a + 1}}}\right|}{c}$$ In this case $$a_{a} = a x$$ and $$x_{0} = 0$$ , $$d = 0$$ , $$c = 1$$ then $$1 = \lim_{a \to \infty}\left(\frac{a}{a + 1}\right)$$ Let's take the limit we find