# Upper bound option price in volatility dimension

All,

I have a theoretical question about the value of an option when spot price goes to infinity as a function of volatility going to infinity.

I know that for a call option:

• The option value equals the discounted payoff when there is zero volatility,

• The option value at infinite volatility is equal to the spot. One can derive this fact by evaluating what happens when $\sigma \to \infty$ in the Black-Scholes PDE.

Now assume we add a dimension of, besides volatility, the spot price.

My question is, given "infinite" spot price, how will the option value approach the spot price by moving 'up' in the volatility dimension?

For example, a (EU) call with (A) strike $100$, spot $100000$, $\tau=1$ year, lets say $r=0$%, and volatility $200$% has value $V=99900.2846$, which is almost equal to payoff. when (B) vola $500$%, $V=99981.6396 \approx$ spot. And for (C) vola $1000$%, $99999.9985$, which is even closer to the exact spot. Now lets say in want to truncate my spot (and volatility) domain and need a boundary condition at the upper bound of this (truncated) spot domain. Is there a way to know how the option value on this bound (spot $\to \infty$, vola $\in (0,\to\infty)$) behaves?

• Am I correct that this is an offshoot from this (quant.stackexchange.com/questions/40123/…) question of yours? I've hinted in my updated (15 Jun) answer for that question as to why your solution doesn't seem to converge to the spot S at your truncated $S_{max}, vol_{max}$ , but you didn't say if you tried my suggestion. All in all I think you're making this more complicated than it is. It's all about applying the boundary conditions correctly. I'd personally wouldn't use any PDE transformation either. – Yian Pap Jun 18 '18 at 17:32
• And as I suggested in that other question, try using $\dfrac{\partial V}{\partial α}=0$ (Neumann) at the max vol boundary and not $V = S$ (Dirichlet). – Yian Pap Jun 18 '18 at 18:17

Let $x$ be a state variable in your model (in your case Finite Difference method in Matlab for SABR volatility model fails to provide correct option values the forward $F$ or the stochastic volatility $\alpha$), then linearity in $x$ on the boundary $x_{\text{min}}$ or $x_{\text{max}}$ means the condition is $\frac{\partial^2V}{\partial x^2} = 0$ on the boundary. Plug that in your pricing PDE and you are left with a PDE on the boundary that has only 1st order derivatives in $x$, which you then approximate using the non centered finite difference $(V_{1,...} - V_{0,...})/\delta x$ or $(V_{i_{\text{max}},...} - V_{i_{\text{max}}-1,...} )/\delta x$. These give you the additional equations that you need to fill in the matrix that represent your discrete linear operator.
• @Antoine_Conze, starting with the PDE, setting $\dfrac{\partial^2 V}{\partial x^2}=0$, and substituting FD approx's that are one-sided in $x$. For the upper bound in $x$ I end up with $V_{i_{max},j-1}^k = [a]V_{i_{max}-1,j-1}^{k+1} + [b]V_{i_{max},j-1}^{k+1} + [c]V_{i_{max}-1,j}^{k+1} + [d]V_{i_{max},j}^{k+1} + [e]V_{i_{max}-1,j+1}^{k+1} + [f]V_{i_{max},j+1}^{k+1}$. How would I proceed to implement this in $A$ in; $V_{i_{min}+1:i_{max}-1,j_{min}+1:j_{max}-1}^{k} = A V_{i_{min}+1:i_{max}-1,j_{min}+1:j_{max}-1}^{k+1}$? – Pim Jun 26 '18 at 14:22
• I don't see an easy implementation besides a very rigorous adjustment of particular matrix elements and corresponding '$C$' vector entries... – Pim Jun 26 '18 at 14:24