Hello, I want to derive the electric field vector from a quadrupole potential. I am starting from faculty.physics.tamu.edu/pope/em603.pdf (6.32) and I am considering E=-∇[itex]\phi[/itex] with [itex]\phi[/itex]=1/2 Qijxixj/r5 I am very unsure if I carry out the thing properly, I would appreciate your comments: Ek=1/2 ∂k(Qij xixj/r5) =Qij∂kxixj/r5 - 5xkxixj/r7 =Qij(δkinj-5nkninj)/r4 with ni=xi/r Now, what is the meaning of δkinj ?! Can it be anyhow simplified? Thanks for reading!