writing statements of research interests

Hello everone.

I'm a master student and applying for PhDs. I need to write a statement of research. I tried to find in this forum about this topic but didn't see any. Does any of you have experiences in writing statements of research? Can you share here? Some questions in my mind about that: what is the main purpose of a statement of research? What is the general structure of it?

Thanks in advance

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Dzung