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Given an aircraft that must visit a sequence of cities, price of fuel in each city, and amount of fuel needed to fly from city to city, and additional fuel burned to carry additional fuel,
How much fuel should be bought at each city?
Qualitatively, we want to buy fuel at the cheap stops, and ferry fuel into the stops where fuel is expensive;

Additional complications might be: city specific landing and takeoff weight limits, nonlinear fuel burn curves, nonlinear fuel purchase schedules at an airport;