I'm reading a journal article at the moment which uses a piece of notation which they dont actually define. It looks like this: [tex]\nabla_{\vec Q}[/tex] (As it happens, [tex]\vec Q[/tex] is an ordinary vector indicating the orientation of a polymer.) I've never seen vector subscript on the gradient symbol "[tex]\nabla[/tex]" before. Could anyone please tell me what [tex]\nabla[/tex] with a vector subscript usually means?
First, since you mention polymers, I am inclined to think the symbol has more to do with chemistry than mathematics. If it really is purely mathematics then I would be inclined to think "the derivative in the direction of [itex]\vec{Q}[/itex]".