Past exam of the mathematics course of the University of Cambridge / 2002 / iii / Paper 32 / 3 / d

The two binary terminal payoffs are complementary:in every state, including $S_T=X$. Long one of each is therefore exactly a unit zero-coupon bond. The law of one price gives digital put-call parity:With deterministic interest, the two separate values are $B(t,T)Q(S_T\ge X\mid {\mathcal{F}}_t)$ and $B(t,T)Q(S_T<X\mid {\mathcal{F}}_t)$. There is no need to assume a continuous terminal price distribution: the strict inequality for the put and weak inequality for the call allocate any atom at the strike exactly once.