Markowitz quickly saw that once risk as well as return was brought into the picture the problem of finding efficient portfolios was not a linear one. The formula for the standard deviation or variance of returns involves squaring rates of return, so the problem could not be solved using existing linear programming techniques: it fell into what was then the little-explored domain of quadratic programming. Koopmans liked Markowitz's term paper, giving it a grade of A, and encouraged him to take the problem further, telling him: "It doesn't seem that hard. Why don't you solve it?" (Markowitz interview) Solve the problem of selecting efficient portfolios was precisely what Markowitz went on to do.

Donald MacKenzie, An Engine, Not a Camera, loc. 211