In this question, the risk-neutral pricing model gives us the following equation to solve:
V_0 = e^(-r(T-t)) * E_Q[S_1^2 | S_0]
I understand we need to derive S_t to resolve S_1^2 any further...
In the next step
dS_t = r * S_t * dt + sigma * S_t * dZ_t ?
What is the reasoning for substituting r in for mu here?
Thanks in advance!