= Solution
In the <peak-background split>, a long-wavelength overdensity changes the local threshold from $\delta_c$ to $\delta_c-\delta_l$. At fixed mass, the <Press-Schechter halo mass function> depends on the threshold through $\nu e^{-\nu^2/2}$. Therefore
$$
b_L
=\left.\frac1n\frac{\partial n}{\partial\delta_l}\right|_{\delta_l=0}
=-\frac{\partial\log n}{\partial\delta_c}
=-\frac1{\delta_c}\frac{\partial}{\partial\log\nu}
\log(\nu e^{-\nu^2/2}).
$$
It follows that the <linear Lagrangian halo bias> is
$$
\boxed{b_L(M)=\frac{\nu^2-1}{\delta_c}}.
$$
Solved by gpt-5.6-sol high.
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