16. If the function $y=f(x)$ for every value $x_{1}$ in its domain, there exists a unique $x_{2}$ in its domain such that $f\left(x_{1}\right) f\left(x_{2}\right)=1$, then the function is called a "dependent function". Given the following propositions:
(1) $y=\frac{1}{x^{2}}$ is a dependent function;
(2) $y=\sqrt{2}+\sin x\left(x \in\left[-\frac{\pi}{2}, \frac{\pi}{2}\right]\right)$ is a dependent function;
(3) $y=2^{x}$ is a dependent function;
(4) $y=\ln x$ is a dependent function;
(5) If $y=f(x)$ and $y=g(x)$ are both dependent functions with the same domain, then $y=f(x) g(x)$ is a dependent function.
Among these, the sequence numbers of all true propositions are
16. (2) (3).

From the definition of dependent functions, we know that in Proposition (1), when $x_{1}=1$, there exists $x_{2}= \pm 1$ satisfying
$$
f\left(x_{1}\right) f\left(x_{2}\right)=1 \text {. }
$$

In Proposition (2), the function is monotonically increasing, with a range of $[\sqrt{2}-1, \sqrt{2}+1],(\sqrt{2}-1)(\sqrt{2}+1)=1$, which meets the requirement.

In Proposition (3), the function is monotonically increasing, with a range of $(0,+\infty)$, which meets the requirement.

In Proposition (4), when $x_{1}=1$, $\ln x_{1}=0$, there does not exist $x_{2}$ satisfying $f\left(x_{1}\right) f\left(x_{2}\right)=1$.

In Proposition (5), if $f(x)=\frac{1}{x}, g(x)=\frac{1}{x}$, Proposition (1) is not satisfied.