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350 Chapter 10 Further Topics<br />

Since<br />

<br />

<br />

<br />

Zt et<br />

α0 <br />

1 +<br />

∞<br />

α j<br />

1e2 t−1 ···e2 <br />

t−j , (10.3.10)<br />

j1<br />

it is clear that {Zt} is strictly stationary and hence, since EZ2 t < ∞, also stationary<br />

in the weak sense. We have now established the following result.<br />

Solution of the ARCH(1) Equations:<br />

If |α1| < 1, the unique causal stationary solution of the ARCH(1) equations is<br />

given by (10.3.10). It has the properties<br />

and<br />

E(Zt) E(E(Zt|es,s

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