European Options (Black-Scholes)
Black-Scholes Parameters →
$$d_1 = \frac{\ln(S_0/X) + (r + \frac{1}{2}\sigma^2)T}{\sigma\sqrt{T}}$$
$$d_2 = d_1 - \sigma\sqrt{T}$$
Digital Options
Digital Call Option →
$$\text{Payoff} =
\begin{cases}
1, & S_T > X \\
0, & S_T \le X
\end{cases}$$
$$\text{Price (today)}: V_{\text{call}} = e^{-rT} N(d_2)$$
Digital Put Option →
$$\text{Payoff} =
\begin{cases}
1, & S_T < X \\
0, & S_T \ge X
\end{cases}$$
$$\text{Price (today)}: V_{\text{put}} = e^{-rT} N(-d_2)$$