In many articles authors describe FMM for particles. e.g. http://www.csci.psu.edu/seminars/fallnotes/fmm.pdf http://www.umiacs.umd.edu/labs/cvl/pirl/vikas/publications/FMM_tutorial.pdf In other articles authors describe FMM for charged surface. How does these formulations relates? Main question: How can I move from FMM formulation for particles to FMM formulation for surfaces? Is it simply by calculating integral(Mnk) instead of Mnk at first step? And then other steps are the same?