NO.PZ2023040502000042
问题如下：
Which of the conclusions regarding the exchange rate time series is consistent with both the properties of a covariance-stationary time series and the properties of a random walk?
Conclusion 1The variance of xt increases over time.
Conclusion 2The mean-reverting level is undefined.
Conclusion 3b0 does not appear to be
significantly different from 0.
选项：
A.Conclusion 1
Conclusion 2
Conclusion 3
解释：
A simple random walk can be described by the equation xt = b0
+ b1xt–1+ εt, where b0 = 0 and b1
= 1. So b0 = 0 is a characteristic of a simple random walk time
series.
A covariance-stationary series must satisfy the following three requirements:
1.The expected value
of the time series must be constant and finite in all periods.
2. The variance of the time series
must be constant and finite in all periods.
3. The covariance of the time series
with itself for a fixed number of periods in the past or future must be constant
and finite in all periods.
b0 = 0 does not violate any of these three
requirements and is thus consistent with the properties of a
covariance-stationary time series.
有单位根的情况下b0是可以等于0(simple random walk)也可以不等于0（random walk with a drift）。另外有单位根的话，mean reversion 确实是不能确认的，这里为什么是选C呢？
