Windjammer

X(t) = b0 + b1*X(t-1)   -----(1)
X(t) = b0 + b1*X(t) ----------(2)

1) to 2) :     t趋向于无穷大的时候，X(t-1) = X(t)
2): X(t) = b0/(1-b1)   X(t) 叫 mean reverting level