If every investor holds T-bills plus the same single fund, why does supply and demand force that fund to be the whole market?
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Two-fund separation: T-bills plus one fund M
State that every mean-variance investor is indifferent between stocks plus T-bills and T-bills plus fund M, list the assumptions, and record how fees and practicality strengthen the case.
Past performers versus mechanical portfolio theory
Record the Buffett objection, the gap between past performance and future skill, and the comparison of estimating covariances against identifying the next genius; note the discount-rate question still left open.
Equilibrium forces M to be the market portfolio
Reproduce the supply-equals-demand argument summing identical risky weights over all investors to get the market-cap-weighted portfolio; name Sharpe, the value-weighted index proxies, and the result's survival of its own popularity.