
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
There are the following four propositions:  
① If $\alpha$ and $\beta$ are both angles in the first quadrant, and $\alpha > \beta$, then $\sin \alpha > \sin \beta$;  
② If the function $y=2\cos(ax- \frac{\pi}{3})$ has a minimum positive period of $4\pi$, then $a= \frac{1}{2}$;  
③ The function $y= \frac{\sin 2x - \sin x}{\sin x - 1}$ is an odd function;  
④ The function $y=\sin(x- \frac{\pi}{2})$ is increasing on the interval $[0,\pi]$;  
Among these, the correct proposition number(s) is/are __________.

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