Why does mixing a stock with T-bills give a straight line instead of a bullet, and why does nobody belong on the bottom half of the curve?
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Stock plus T-bills:
Show how the variance and covariance terms vanish, work one row from 0.12% and 0.75% with 4.25% volatility, and draw the straight line including the borrowed segment.
Leverage with unstable correlations in 2008
Record how levering far up the line combined with correlations jumping toward 1 produced wipeouts, and the error of treating an estimated correlation as a physical constant.
The efficient frontier
Define it as the upper half of the minimum-variance boundary, state the argument that rules out interior and lower-half points, and record what the theory leaves to risk preferences.