Hi, Just curious: what is the equation of a plane in R^n for n>3 ? We cannot define a normal vector for n>3 , so, what do we do? I thought of working with flat, embedded copies of R^2 . In terms of linear algebra, I guess we could see a plane in R^n as any image of a linear map with rank=2. In terms of geometry, maybe we need a "flat" embedding of R^2 (I am not clear how to make the term 'flat' here more precise) in R^n. Anyone know? Thanks.