
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
In the regular tetrahedron P-ABC, D is the midpoint of PA, and O is the centroid of triangle ABC. Given the following four conclusions:  
① OD is parallel to plane PBC;  
② OD is perpendicular to PA;  
③ OD is perpendicular to BC;  
④ PA = 2OD.  
Among these conclusions, the correct ones are.

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