
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 definition: If the function $f(x)$ is differentiable on $D$, meaning $f'(x)$ exists, and the derivative function $f'(x)$ is also differentiable on $D$, then $f(x)$ is said to have a second-order derivative on $D$, denoted as $f''(x)=(f'(x))'$. If $f''(x)<0$ holds true on $D$, then $f(x)$ is called a concave function on $D$. Among the following four functions, identify the ones that are not concave on the interval $(0, \frac{\pi}{2})$. (Fill in all the correct serial numbers)  
① $f(x)=\sin x+\cos x$;  
② $f(x)=\ln x-2x$;  
③ $f(x)=-x^3+2x-1$;  
④ $f(x)=xe^x$.

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