Given the price of a call equals to 5 with Strike 100, please find the upper bound (sup) of the digital option with strike 105.
I am not sure about the solution, but I write the condition like this,
$S\mathcal{N}(d_1)-Ke^{-rT}\mathcal{N}(d_2) = 5$
what's the $\sup{N(d_1+\frac{\ln(\frac{100}{105})}{\sigma\sqrt{T}})}$?