What is a set?
A set groups well-defined objects; membership, inclusion and operations describe its structure.
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Graphs, degrees, and Eulerian trails
In a multigraph whose vertices of nonzero degree are all connected, degree parity determines whether every edge can be traversed exactly once.
The law of diminishing returns
With technology unchanged, increasing one input while the others remain fixed often ultimately reduces its marginal product; if it is the only variable input, paid at a constant price, and as long as its marginal product remains positive, this decline raises marginal cost.
Comparative advantage
In the two-good, constant-cost model, reallocation follows opportunity cost; if preferences are strictly increasing in chairs over the bundles compared, a price strictly between the two costs enables mutual gain.
The time value of money
An amount means nothing without its date. Comparing sums placed at different dates requires bringing them back to a single date: a flow weighs the less the more distant its maturity, as soon as the reference return is positive.
Compound interest
When interest is added to the principal, it in turn earns interest: the sum no longer grows by a fixed amount each period, it is multiplied by a fixed factor.
Supply and Demand
Supply and demand are not quantities but two relations between price and quantity; their intersection defines the equilibrium price.
Opportunity Cost
The cost of a choice is not limited to the money spent: it is the value of the best option it makes you give up.
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