It should be apparent that the movement of balance sheets (in the form of income sheets) can be modeled approximately by (discrete and/or discontinuous) partial

It should be apparent that the movement of balance sheets (in the form of income sheets) can be modeled approximately by (discrete and/or discontinuous) partial differential equations. The fundamental principle is rate of change of the balance sheet is relatively proportional to the amount present. Qualitatively, "It takes money to make money." Thus, a homeless person getting from $0 to $1M cannot be had without some range of help of others and a person growing at roughly $100M to $101M shouldn't be as difficult per se. Corollary: If you can count your money, then you're not rich. (Liquid net worth is always an educated guess, varies by kinds of assets and debts, liquidation timeframe, and varies by the day.)