Definition 60.9.1. In Situation 60.7.5.

The (small)

*crystalline site*of $X$ over $(S, \mathcal{I}, \gamma )$, denoted $\text{Cris}(X/S, \mathcal{I}, \gamma )$ or simply $\text{Cris}(X/S)$ is the full subcategory of $\text{CRIS}(X/S)$ consisting of those $(U, T, \delta )$ in $\text{CRIS}(X/S)$ such that $U \to X$ is an open immersion. It comes endowed with the Zariski topology.The topos of sheaves on $\text{Cris}(X/S)$ is denoted $(X/S)_{\text{cris}}$ or sometimes $(X/S, \mathcal{I}, \gamma )_{\text{cris}}$

^{1}.

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