Should I ask colleagues of my mentor for internship opportunity? I have a mentor from an industrial research lab. I use the tool he has developed in my PhD research, and we have been working together for several months. We have some results that he likes very much, since my work is a new application of his tool. However, unfortunately our paper has been rejected two times.

About a month ago, I saw an ads of internship positions at his group, so I asked him about the position. He hadn't replied anything until some days ago, he replied that this year he is busy organizing a (top-tier) conference, so he was reluctant to get students.

The ads is still there, and I still want to apply. Should I ask his colleagues if they are accepting students? More specific questions:


Should I ask my mentor which his colleagues are accepting students?
Should I ask my mentor's colleagues, there are 4 of them, without notifying him?
If I contact my mentor's colleagues, should I mention his name.


My worries are about the reference that my mentor can give to me. My impression is that he likes my work, but since the paper is rejected two times, I don't know that if it is good. I also don't know if it is rude asking to work with others while working with him.

My mentor is an established researcher with great personality, he treats unknown students like me just like his friends or colleagues. I'm very happy working with him, and I don't want anything that may damage our collaboration.
I think it would be perfectly reasonable for you to a) ask your mentor which of his colleagues might be accepting students and b) mention his name when contacting his colleagues, AFTER you've first made sure he's ok with this plan. 

I don't think you need to be worried about a conflict: he's opened the door by saying that he's too busy to take on students this summer. As to whether he'd give you a good reference, that's quite hard to say, which is why it's best to ask him. Paper rejection in and of itself doesn't mean he doesn't like your work.