$\begin{aligned}
& f(f(x))=\frac{2 f(x)}{\sqrt{1+9 f^2(x)}}=\frac{4 x}{\sqrt{1+9 x^2+9.2^2 x^2}} \
& f(f(f(x)))=\frac{2^3 x / \sqrt{1+9 x^2}}{\sqrt{1+9\left(1+2^2\right) \frac{2^2 x^2}{1+9 x^2}}}=\frac{2^3 x}{\sqrt{1+9 x^2\left(1+2^2+2^4\right)}}
\end{aligned}\thereforeByobservation\begin{aligned}
& \alpha=1+2^2+2^4+\ldots .+2^{18}=1\left(\frac{\left(2^2\right)^{10}-1}{2^2-1}\right)=\frac{2^{20}-1}{3} \
& 3 \alpha+1=2^{20} \rightarrow \sqrt{3 \alpha+1}=2^{10}=1024
\end{aligned}$