a（L）到底等于什么呢？

问题如下：

What are the properties of the lag operator?

解释：

a. The lag operator shifts the time index back one observation.

b. Lag operators multiply so that $LL = L^2$ and $L^2Y_t = L(LY_t) = LY_{t - 1}$.

c. The lag operator applied to a constant is just the constant, $L\delta = \delta$.

d. The pth order lag polynomial is written $\phi (L) = 1 + \phi_1 L + \phi_2L^2 + ... + \phi_pL^p$.

e. Lag polynomials can be multiplied to produce another lag polynomial.

f. Under some assumptions, lag polynomials can be inverted.