Digital put-call parity
= Digital put-call parity
Complementary digital call and put payoffs sum to one, so their time-$t$ values satisfy
$$
D_{\rm call}(t,S)+D_{\rm put}(t,S)=e^{-r(T-t)}.
$$
= Digital put-call parity
Complementary digital call and put payoffs sum to one, so their time-$t$ values satisfy
$$
D_{\rm call}(t,S)+D_{\rm put}(t,S)=e^{-r(T-t)}.
$$