# Put pricing embedded in autocall

When pricing an autocall, there are 3 parts:

1. Strip of coupon,
2. Zero coupon bond,
3. Put down and in.

Probabilities of a call is given from the trigger level on call dates. However, let's say my autocall is 1Y max and is callable on months 6 and 12, should I price 2 put D&I and multiply the price by the autocall probabilities ? Or should I just compute the 1Y Put D&I ?

The reason is: When pricing a down and in put with Heston model I'm in line with what's expected. However, when moving to autocall products, my coupons are below what are priced by bank's pricers

About my computation: Given Monte Carlo paths and trigger level, I compute probabilities of touching the trigger. This gives me a set of probabilities for each call dates. I compute then discount factors * ZC bond value and this gives me my expected ZC bond value given my probabilities. In the end, I sum up the price of the put and (1 - ZC Present value). Finally I divide the result by the ZC bond expected value. Is it correct ?

Edit: what i am looking for if given the put price, my probabilities, my discount factors, i want to get the guaranteed coupons. So this is not exactly the same as getting the payoff and computing the value of the product.. I'm solving the equation Present Value of contingent coupon (probabilized) + PV of probabilized zero coupon - Put = 0. Some help would be appreciated, thanks

## 1 Answer

I'm not sure I correctly understood your question. Consider this simple autocall structure. Let $T_1$ denote the observation date of the autocall feature. More specifically, if the underlying spot price at $T_1$ is higher than a given threshold $\alpha$, the structure will pay a coupon $C$ to its holder and then expire. Otherwise, the structure will deliver the same cash flow as a down and in put with maturity $T_2 > T_1$.

Assuming zero discount rates for the sake of clarity, the price of the aforementioned structure is

\begin{align} V_t &= \Bbb{E}_t^\Bbb{Q}\left[ 1\{ S_{T_1} \geq \alpha \} C + 1\{ S_{T_1} < \alpha \} \phi(S_{T_2}) \right] \\ &= \Bbb{E}_t^\Bbb{Q}\left[ 1\{ S_{T_1} \geq \alpha \} C \right] + \Bbb{E}_t^\Bbb{Q}\left[ 1\{ S_{T_1} < \alpha \} \phi(S_{T_2}) \right] \end{align}

Now, although you can always write $$\Bbb{E}_t^\Bbb{Q}\left[ 1\{ S_{T_1} \geq \alpha \} C \right] = C \,\, \underbrace{\Bbb{Q}(S_{T_1} \geq \alpha)}_{\text{call prob.}}$$ you can on the other hand only write $$\Bbb{E}_t^\Bbb{Q}\left[ 1\{ S_{T_1} < \alpha \} \phi(S_{T_2}) \right] = \underbrace{\Bbb{E}_t^\Bbb{Q}\left[ 1\{ S_{T_1} < \alpha \} \right]}_{1-\text{call prob.}} \underbrace{\Bbb{E}_t^\Bbb{Q}\left[ \phi(S_{T_2}) \right]}_{\text{DI put price}}$$ iff the random variables $1\{ S_{T_1} < \alpha \}$ and $\phi(S_{T_2})$ are independent, which is not the case in most diffusion models where $S_{T_1}$ and $S_{T_2}$ are dependent.

Wrapping up, this means that in general, you cannot write that: $$\text{autocall price} = \pi \, \text{coupon} + (1-\pi) \, \text{down and in put price}$$ where $\pi$ is here represents the autocall probability at a single observation date.

• I'm expecting the price of the structure to be flat i.e. Long Coupon long ZC short Put = 100%. Maybe that's what I'm also misunderstanding. My method is the following: I first compute the price of the Put with maturity the one of the structure. My unknown variable is the coupon rate by period that equals my flows. Next i compute my ZC discounted and probabilized according to my model (contingent proba on each call dates). Finally I compute coupons but i think that's on this last step that i mix up things. One of the questions i had is also, say i have 4 call dates, should i compute 4 puts? – Cedric_W Jun 5 '18 at 9:10
• My goal is really to find the periodic coupon, not to find the price given a coupon rate. I have several elements needed for the pricing (put, ZC etc but I'm unsure regarding the methodology to get the Coupon) – Cedric_W Jun 5 '18 at 9:12
• If you have a pricer then why don't you solve for the periodic coupon numerically ? – Quantuple Jun 5 '18 at 9:19
• I have the pricer and i have the coupon from it indeed, but that's a black box. I need to replicate it and now I'm checking my results and investigating gaps. – Cedric_W Jun 5 '18 at 9:22