Solved! Hi all, I'm trying to work out the following question: I know to be solenoidal the divergence of the vector must be 0, however I am having a huge amount of trouble understanding how to differentiate this. I don't know when to treat things as a vector and when to treat them as a scalar. Some further information: a is an arbitrary, constant vector. r is the position vector xi + yj + zk, and r is the magnitude of this vector. So, to emphasise: my problem is when to treat things as a vector and when to treat them as a scalar. I know bold denotes a vector and the dot product of a.r turns that part into a scalar - but I'm lost after that. I've been working on this question for about three hours so I'd really appreciate any help anyone can offer.