formula of demand

[aRNoLD]

willingness+purchasing power-competitor's share=demand

in lecturing, we are told demand are from customers' buying willingness and their (potential) purchasing power, but i share my view here and propose a newer formula above.

in a fixed market, a company is operating in it with a few direct rivals, and they are surely able to, and definately will grab your share in the market, thus results a loss/reduction of the company's share/sales volumn. This can be deducted from the classic formula----willingness+purchasing power = demand.