
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
Among the following functions $f(x)$, which satisfies "For any $x_1, x_2 \in (0, +\infty)$, when $x_1 < x_2$, it always holds that $f(x_1) < f(x_2)$"? (Fill in the serial number)  
① $f(x) =$ ;  
② $f(x) = (x-1)^2$;  
③ $f(x) = e^x$;  
④ $f(x) = \ln(x+1)$.

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