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There is a professor who has made several reference letters for me, and I would like to thank him. However, I have some doubts as to whether it would at all be appropriate to thank him with dinner delivery to his office, say. I feel that simple verbal thanking is not sufficient at this point, but then, the atmosphere of formality is still there at the level of university relationships. What are one's ideas on this? If sending him a fine dinner would not be appropriate, especially without first discussing this with him, what is another way of thanking a professor, which would be considered "formally appropriate"?