M æ P t x ö Q q § S w M ô h Å · Wu P, Hao X, Lau EHY, et al Realtime tentative assessment of the epidemiological characteristics of novel coronavirus infections in Wuhan, China, as at January Eurosurveillance ;25(3The position vector of a point P(x,y) in two dimensions is xi yj We will often denote this important vector by r See Diagram 2 (In three dimensions the position vector is r = xiyj zk) x y 0 i j Diagram 1 x y 0 xi yj P(x,y) r Diagram 2 Section 1 Introduction (Vectors) 4 The vector differential operator ∇, called "del" or "nabla", is defined in three dimensions to be Report Submitted In Partial Fulfillment Of The Requirement For The X^[EH[Y ¢ç·Æâ