Definition 42.29.1. Let $(S, \delta )$ be as in Situation 42.7.1. Let $X$ be locally of finite type over $S$. Let $(\mathcal{L}, s)$ be a pair consisting of an invertible sheaf and a global section $s \in \Gamma (X, \mathcal{L})$. Let $D = Z(s)$ be the zero scheme of $s$, and denote $i : D \to X$ the closed immersion. We define, for every integer $k$, a *Gysin homomorphism*

by the following rules:

Given a integral closed subscheme $W \subset X$ with $\dim _\delta (W) = k + 1$ we define

if $W \not\subset D$, then $i^*[W] = [D \cap W]_ k$ as a $k$-cycle on $D$, and

if $W \subset D$, then $i^*[W] = i'_*(c_1(\mathcal{L}|_ W) \cap [W])$, where $i' : W \to D$ is the induced closed immersion.

For a general $(k + 1)$-cycle $\alpha = \sum n_ j[W_ j]$ we set

\[ i^*\alpha = \sum n_ j i^*[W_ j] \]If $D$ is an effective Cartier divisor, then we denote $D \cdot \alpha = i_*i^*\alpha $ the pushforward of the class $i^*\alpha $ to a class on $X$.

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