### 27 Black Scholes PDE

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#### 27.1 classic Black Scholes model from ﬁnance, European call version

problem number 163

From Mathematica symbolic PDE document.

Solve for $$V(S,t)$$ where $$V$$ is the price of the option as a function of stock price $$S$$ and time $$t$$. $$r$$ is the risk-free interest rate, and $$\sigma$$ is the volatility of the stock.

$\frac{\partial V}{\partial t} + \frac{1}{2} \sigma ^2 S^2 \frac{\partial ^2 V}{\partial S^2} = r V - r S \frac{\partial V}{\partial S}$

With boundary condition $$V(S,T) = \max \{ S-k,0 \}$$

Reference https://en.wikipedia.org/wiki/Black%E2%80%93Scholes_equation See the European call version at bottom of the page.

Mathematica

$\left \{\left \{u(x,t)\to \frac{1}{2} k e^{\frac{\sigma ^2 t}{2}+x-1} \left (\text{Erf}\left (\frac{\sigma ^2 t+x}{\sqrt{2} \sqrt{t} \left | \sigma \right | }\right )+1\right )\right \}\right \}$

Maple

$u \left ( x,t \right ) =-k{{\rm e}^{-1}} \left ( i{\it invfourier} \left ({\frac{{{\rm e}^{-1/2\,{s}^{2}{\sigma }^{2}t}}}{s+i}},s,x \right ) -{\it invfourier} \left ({{\rm e}^{-1/2\,{s}^{2}{\sigma }^{2}t}}{\it fourier} \left ({{\rm e}^{x}},x,s \right ) ,s,x \right ) \right )$

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#### 27.2 Boundary value problem for the Black Scholes equation

problem number 164

From Mathematica DSolve help pages.

Solve for $$V(t,s)$$

$\frac{\partial v}{\partial t} + \frac{1}{2} \sigma ^2 s^2 \frac{\partial ^2 v}{\partial s^2} +(r-q) s \frac{\partial v}{\partial s} - r v(t,s)=0$

With boundary condition $$v(T,s) = \psi (s)$$

Mathematica

$\left \{\left \{v(t,s)\to \frac{e^{r (t-T)} \int _{-\infty }^{\infty } \psi \left (e^{K[1]}\right ) \exp \left (-\frac{\left (-K[1]+(T-t) \left (-q+r-\frac{\sigma ^2}{2}\right )+\log (s)\right )^2}{2 \sigma ^2 (T-t)}\right ) \, dK[1]}{\sqrt{2 \pi } \sqrt{\sigma ^2 (T-t)}}\right \}\right \}$

Maple

$v \left ( t,s \right ) =\psi \left ( s \right ) +\sum _{n=1}^{\infty }{\frac{ \left ( t-T \right ) ^{n} \left ( U\mapsto rU^{ \left ( n \right ) } \right ) \left ( \psi \left ( s \right ) \right ) }{n!}}$