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Baskets [of securities] form a vector space…. A portfolio is … an equivalence class of baskets containing assets in the same proportions.… [Since] parameters … depend on proportions … rather than … actual values,… portfolios form a projective space.

Edward W. Piotrowski and Jan Sładkowski, Geometry of Financial Markets

In fact this could be said of any economic price ratios, in or outside finance.

When history trips the threshold of price space into the region where farm productivity > cost of plow & oxen > wealth of five families, then villages happen.

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