Apa itu COS method?

The COS method (Fang and Oosterlee, 2008) truncates the density of $y = \ln(S_T/K)$ to an interval $[a, b]$, expands it in a cosine series whose coefficients are read off the characteristic function, $A_k \approx \frac2{b-a}\mathrm{Re}[\varphi(\frac{k\pi}{b-a})e^{-\mathrm ik\pi a/(b-a)}]$, and integrates the payoff against each cosine in closed form, so that a price is $e^{-rT}\sum_{k<N}{}'A_kV_k$ (the first term halved).