<P> Mathematically, the inverse relationship described by the law of demand may be expressed as: </P> <Dl> <Dd> Q x = f (P x; Y), ∂ f / ∂ P x <0, (\ displaystyle Q_ (x) = f (P_ (x); (\ mathbf (Y))), \ quad \ partial f / \ partial P_ (x) <0,) </Dd> </Dl> <Dd> Q x = f (P x; Y), ∂ f / ∂ P x <0, (\ displaystyle Q_ (x) = f (P_ (x); (\ mathbf (Y))), \ quad \ partial f / \ partial P_ (x) <0,) </Dd> <P> where Q x (\ displaystyle Q_ (x)) is the quantity demanded of good x (\ displaystyle x), P x (\ displaystyle P_ (x)) is the price of the good, f (\ displaystyle f) is the demand function, ∂ f / ∂ P x (\ displaystyle \ partial f / \ partial P_ (x)) is the partial derivative of the demand function with respect to P x (\ displaystyle P_ (x)), and Y (\ displaystyle (\ mathbf (Y))) is the list of other parameters held constant . </P>

When does the law of demand not apply