<P> a differential 2 - form is the formal sum: </P> <Dl> <Dd> a 12 d x ∧ d y + a 13 d x ∧ d z + a 23 d y ∧ d z; (\ displaystyle a_ (12) \, dx \ wedge dy + a_ (13) \, dx \ wedge dz + a_ (23) \, dy \ wedge dz;) </Dd> </Dl> <Dd> a 12 d x ∧ d y + a 13 d x ∧ d z + a 23 d y ∧ d z; (\ displaystyle a_ (12) \, dx \ wedge dy + a_ (13) \, dx \ wedge dz + a_ (23) \, dy \ wedge dz;) </Dd> <P> and a differential 3 - form is defined by a single term: </P>

What does the curl of a vector field tell us