# Cox-Ingersoll-Ross Zero Bond Put Option

according to Brigo & Mercurio (2006): But how is the Zero bond Put of the CIR model? I couldn't find any information about that.

You can simply apply formula (3.4) in Brigo and Mercurio's book (page 56). There is a simple put-call parity for the prices of European-style options written on zero-coupon bonds, i.e. \begin{align*} \mathbf{ZBP}(t,T,S,X) = \mathbf{ZBC}(t,T,S,X) -P(t,S)+XP(t,T). \end{align*} The formula is kind of identical to the standard equity put call parity where you discount the strike $$X$$ with the discount factor $$P(t,T)$$ and where $$P(t,S)$$ is the price of the underlying asset (a bond itself). It also follows directly from the Law of One Price (or no-arbitrage). Based on this observation, there are many parities relating interest rate derivatives (e.g. caps and floors as well as payer swaptions and receiver swaptions).
Please also note that the zero-coupon bond formula follows the 'Black-Scholes-style' \begin{align*} \mathbf{ZBC}(t,T,S,X) = P(t,S)\Pi_1 - XP(t,T) \Pi_2, \end{align*} where $$\Pi_1$$ and $$\Pi_2$$ are two probabilities (here related to the $$\chi^2$$ distribution of the short rate in the CIR model). Thus, the price of a zero-coupon bond put option equals \begin{align*} \mathbf{ZBP}(t,T,S,X) = XP(t,T) (1-\Pi_2) - P(t,S)(1-\Pi_1). \end{align*} The probabilities $$\Pi_1$$ and $$\Pi_2$$ can be seen as exercise probabilities using a change of numeraire (i.e. using the bonds maturing at time $$T$$ and $$S$$ as numeraires).
• @ChristianM I'm not quite sure I get your question. A floor is a portfolio of European-style zero-coupon bond call options. The bond call options can be prices with the formula cited in your question. The floor price is then a linear combination of these bond option prices. The corresponding cap price can be found by the cap floor parity. The $\Pi_1$ and $\Pi_2$ in my answer correspond to $N(d_1)$ and $N(d_2)$ in the Black-Scholes equity options case or to $\chi^2(a,b,c)$ in the CIR model (see your question). They are just probabilities. Nov 13 '19 at 20:02
• @ChristianM Question fully answered and no open questions left? I hope I didn't confuse you with the comments about $\Pi_1$ and $\Pi_2$. It's just nice to see how these formulae fit into a 'greater picture' (: Nov 13 '19 at 20:05