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There are two common explanations for upward sloping yield curves.
This function Y is called the yield curve, and it is often, but not always, an increasing function of t.
In finance, the yield curve is a curve showing several yields or interest rates across different contract lengths (2 month, 2 year, 20 year, etc. The curve shows the relation between the (level of the) interest rate (or cost of borrowing) and the time to maturity, known as the "term", of the debt for a given borrower in a given currency. With other factors held equal, lenders will prefer to have funds at their disposal, rather than at the disposal of a third party.