<P> The supply function is the mathematical expression of the relationship between supply and those factors that affect the willingness and ability of a supplier to offer goods for sale . An example would be the curve implied by Q s = f (P; P rg) (\ displaystyle Q_ (\ text (s)) = f (P; P_ (\ text (rg)))) where P (\ displaystyle P) is the price of the good and P rg (\ displaystyle P_ (\ text (rg))) is the price of a related good . The semicolon means that the variables to the right are held constant when quantity supplied is plotted against the good's own price . The supply equation is the explicit mathematical expression of the functional relationship . A linear example is Q s = 325 + P − 30 P rg (\ displaystyle Q_ (\ text (s)) = 325 + P - 30P_ (\ text (rg))). Here 325 (\ displaystyle 325) is the repository of all non-specified factors that affect supply for the product . The coefficient of P (\ displaystyle P) is positive following the general rule that price and quantity supplied are directly related . P rg (\ displaystyle P_ (\ text (rg))) is the price of a related good . Typically its coefficient is negative because the related good is an input or a source of inputs . </P> <P> The relationship of price and supply curve . The curve is generally positively sloped . The curve depicts the relationship between two variables only; price and quantity supplied . All other factors affecting Supply are held constant . However, these factors are part of the supply equation and are implicitly present in the constant term . </P> <P> Movements along the curve occur only if there is a change in quantity supplied caused by a change in the good's own price . A shift in the supply curve, referred to as a change in supply, occurs only if a non-price determinant of supply changes . For example, if the price of an ingredient used to produce the good, a related good, were to increase, the supply curve would shift left . </P> <P> By convention in the context of supply and demand graphs, economists graph the dependent variable (quantity) on the horizontal axis and the independent variable (price) on the vertical axis . The inverse supply equation is the equation written with the vertical - axis variable isolated on the left side: P = f (Q) (\ displaystyle P = f (Q)). As an example, if the supply equation is Q = 40 P − 2 P r g (\ displaystyle Q = 40P - 2P_ (rg)) then the inverse supply equation would be P = Q 40 + P r g 20 (\ displaystyle P = (\ tfrac (Q) (40)) + (\ tfrac (P_ (rg)) (20))). </P>

12. name three factors other than price that might affect the supply of an item