
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given the functions ① $f(x)=x^2$; ② $f(x)=e^x$; ③ $f(x)=\ln x$; ④ $f(x)=\cos x$. Among them, for any $x_1$ within the domain of $f(x)$, there exists a unique $x_2$ such that $f(x_1)f(x_2)=1$ holds. Find the function(s) that meet this condition.

Present the answer in LaTex format: \boxed{Your answer}
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