# Gil-Palaez Inversion Formula in Black Scholes world

I am trying to calculate numerically the price of a plain vanilla call through Fourier Transform, by applying the Gil-Pelaez formula. More precisely, we have that

$$C(K) = S_0 \Pi_1 - K e^{-r T} \Pi_2,$$

where

\begin{eqnarray} \Pi_1 & = & \frac{1}{2} + \frac{1}{\pi} \int_0^\infty \mathfrak{Re} \left\{ \frac{\phi(u-i) e^{-\mathrm{i} u \ln(K)}}{\phi(-\mathrm{i}) \mathrm{i}u} \right\} \mathrm{d}u,\\ \Pi_2 & = & \frac{1}{2} + \frac{1}{\pi} \int_0^\infty \mathfrak{Re} \left\{ \frac{\phi(u) e^{-\mathrm{i} u \ln (K)}}{\mathrm{i} u} \right\} \mathrm{d}u \end{eqnarray}

and where

$$\phi(u) = \exp \left\{ \mathrm{i} \left( \ln \left( S_0 \right) + \left( r - \frac{1}{2} \sigma^2 \right) T \right) u - \frac{1}{2} \sigma^2 u^2 T \right\}.$$

Although I do the algebra, it seems that that the integrands I get inside $\Pi_1$ and $\Pi_2$ are the same, something which is obviously false. Although I know it's a stupid question, could you please help me on which are the real parts of the integrands? You help is highly appreciated since I have tried calculating them several times.

• Could you please use latex? – Ric Oct 12 '15 at 18:18

Both integrand are different. One includes $$\phi(u-i)$$ and the other one simply $$\phi(u)$$. As one expects, in the Black-Scholes model, $$\Pi_1$$ and $$\Pi_2$$ collaps to $$\Phi(d_1)$$ and $$\Phi(d_2)$$.
Note firstly that if $$X\sim N(\mu,\sigma^2)$$, then \begin{align*} \phi_X(u) &= e^{iu\mu-\frac{1}{2}\sigma^2u^2}, \\ \phi_X(u-1) &=\phi_X(u) e^{\mu+\frac{1}{2}\sigma^2}e^{iu\sigma^2},\\ \phi_X(-i) &= e^{\mu+\frac{1}{2}\sigma^2}, \\ \frac{\phi_X(u-i)}{\phi_X(-i)} &= \phi_{\tilde{X}}(u), \end{align*} where $$\tilde{X}\sim N(\mu+\sigma^2,\sigma^2)$$. Thus, \begin{align*} \Pi_1 &= \frac{1}{2}+\frac{1}{\pi}\int_0^\infty \Re\left(\frac{e^{-i\ln(K)u}\varphi_{\tilde{\ln(S_T})}(u)}{iu}\right)\mathrm{d}u \\ &= 1-F_{\tilde{\ln(S_T)}}\big(\ln(K)\big) \\ &= 1-\Phi\left( \frac{\ln(K)-\left(\ln(S_0)+\left(r-q-\frac{1}{2}\sigma^2\right)T \right)- \sigma^2T}{\sigma\sqrt{T}}\right) \\ &= 1-\Phi\left( -\frac{\ln\left(\frac{S_0}{K}\right)+\left(r-q+\frac{1}{2}\sigma^2\right)T }{\sigma\sqrt{T}}\right) \\ &=1-\Phi(-d_1) \\ &=\Phi(d_1). \end{align*}
The second line applies the Gil-Pelaez formula which reads as follows $$F_X(x) = \frac{1}{2}-\frac{1}{\pi} \int_0^\infty \Re\left(\frac{e^{-iux}\phi_X(u)}{iu}\right)\mathrm{d}u.$$
The case for $$\Pi_2$$ is the same and you can recover $$\Phi(d_2)$$.