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After writing the first draft of my paper, I sent the proof to an expert in my area. He responded my email in a neutral way, something like: I think if your argument is true then it would be nice,... I suggest you may read the paper ... to ...
Now, I would like to acknowledge him in my paper, sth. like: "I would like to thank Professor A for his comments". In my opinion, I should do this to show my gratitude. Besides, this also makes my paper somehow more concrete and professional, I think so.
My question: Do I need to ask Professor A the permission to acknowledge him? Or I just acknowlegde him without asking?