Dot product! Note that "Rn" is R^1000 here.
suppose you manage a mutual fund that invests in 1000 companies. Let S be the vector in Rn whose ith component is the number of shares of company i that you've today. Let P be the vector in R^1000 whose ith component is today's price per share of company i's stock. Express the total value of your holdings in terms of vector operations.