Hello,
I found that the definition of the double layer on the “Bempp book” Bempp book is not matched with the publication(E. V. Wout, P. Gelat, T. Betcke and S. Arridge, 138(5), JASA, 2015). I have checked the Bempp code, and found that it might be consistent with the later definition from JASA paper. The two definitions are parsed below. I hope my finding is useful for other users.
[Bempp book]
JASA Paper