利率：取决于违约风险
无风险利率：risk-free。仅指无违约风险。no default risk or credit risk, still have market risk
种类：
1. Treasury rates
2. LIBOR: London Interbank Offered Rate
3. SHIBOR: Shanghai Interbank Offered Rate
4. Repo: repurchase agreement rates
连续复利：近似于每日复利
Rc连续复利利率，Rm每年复利m次