Unforgeability proof of partially blind signature (schnorr signature based)
If the probability (over random choice of the random tape $\rho$ and the random oracle $\mathcal{H}$) of a particular trial being successful is at least $\mu'$, then the probability that $1/\mu'$ independent trials* are unsuccessful is at most $(1-\mu')^{1/\mu'}$ --- this expression converges to $1/e$. The claim follows.

*Here, independence refers to the choice of $\rho$ and $\mathcal{H}$.