Short note on ways to avoid negative short rate

- BK, BLT or other exponential-Gaussian models
- Imposing absorbing or reflecting BC (Goldstein and Keirstead)
- Kim and Singleton reviewed a few possibilities:

• Affine models like CIR, under which zero is not accessible
• Quadratic Gaussian models
• Black's shadow rate (i.e. treating bond yield as a call option to a latent process that CAN go negative)