大家好，我现在在美国留学，在准备考CFA2级。刚注册这个论坛，希望能向大家学习。以下是我遇到的几个问题，还请不吝赐教。

PS: 我的qq是1214877519，有兴趣讨论的同学我们也可以用qq联系。

1. 请问single-name CDS的payoff为什么是“based on the market value of the cheapest-to-deliver bond that has thesame seniority as the reference obligation”？为什么不是具有相同seniority的所有bonds的market values的平均值？

2．Kaplan 2014年CFA二级的Study Notes中（Book5的141页）在讲完CDS spread的计算方法之后，马上又说了一句：“We can also quotethe CDS price as: price of CDS (per $100 notional) = $100 - upfront premium。”

在我的印象当中，我一直以为在买卖CDS的当时，买卖双方只支付upfront premium（如果这是个正数，则买方付给卖方；如果是负数，则卖方付给买方）；而在买卖完成之后，则是买方给卖方支付CDS coupon。所以我不理解为什么还需要引入这个“CDS price”的概念？这个“CDS price”是哪一方付给哪一方的？是在买卖CDS的当时还是买卖之后支付的？

3. Suppose thatthe process followed by the underlying stock price in a risk-neutral world is

dS = u*S*dt +sigma*S*dz,

where dz is aWiener process, u is the expected return in a risk-neutral world, and sigma is thevolatility.
To simulate the path followed by S, we can divide thelife of the derivative into N short intervals of length Delta_t and approximatethe above equation as

S(t + Delta_t) -S (t) = u*S(t)*Delta_t + sigma*S(t)*epsilon*SquareRoot(Delta_t),

where epsilon isa random sample from a standard normal distribution.

我想确认一下：这里之所以用根号Delta_t，是为了保证sigma*S(t)*epsilon*SquareRoot(Delta_t)的方差约等于sigma*S*dz的方差，对吗？