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M <br />
N β i<br />
<br />
<br />
N = 1 β i = 0. <br />
τ <br />
t <br />
<br />
<br />
<br />
<br />
<br />
<br />
<br />
<br />
N = 1 <br />
<br />
<br />
<br />
<br />
<br />
<br />
<br />
N <br />
<br />
<br />
<br />
<br />
∇ µ ∇µt = 0. <br />
N <br />
<br />
∂<br />
− Lβ<br />
∂t<br />
<br />
N = −KN 2 .