生存分析样本量计算

two arm randomized clinical trials, 计算所需event数量。power=0.8，one-sided type I = 0.25，effect size = 1.75， in terms of ratio of mean lifetimes（不是median lifetime）. Lognormal distributed times in both treatment and control. Equal sample allocation.
其他所需的前提条件（比如event rate等）均可自由设定。
求大神帮忙指点，具体的计算公式是如何推导的，越详细越好