Collaboration and its Effect on Tenure and Promotion?
Letters of Recommendation and Paper Counting
* that you are a mature and independent researcher,
* and that you will continue to do research of sufficient quality and quantity.
Policies about whether collaborators may write recommendation letters on your behalf vary with institutions. You should inquire about what the policy at your institution is. If they have a policy not to consider recommendations from collaborators, you should make sure that your work is known by others in the field besides your collaborators. Send your preprints to others in the field and ask for comments. That way there will be non-collaborators who have an idea of what you are doing.
It is important also to consider where your papers appear. it is best to have papers in standard refereed journals, not only conference proceedings. While papers in specialized journals will count, papers in wider range journals, for example, Transactions AMS or Proceedings AMS or other such may often count more. A good rule of thumb is that fewer papers in better journals counts more than lots of papers in less known journals or proceedings.