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Probability of default under Merton


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The Black-Scholes option pricing formula has d1=ln(S/K)+.... Yet, the Stulz reading and in the notes, has "BSM risk-neutral d2 is: d2=ln (S/K...) (p.8 of the notes). Should be not d1 and I understand replacing the risk-free for the mean drift. Trying to understand why we move from d1 to d2 by replacing the risk-free rate for the drift rate.