Definition 11.1.4 (Invertible object). An object \(X\) of \(C\) is called invertible if there exists another object \(X^{-1}\) and an isomorphism \(X \otimes X^{-1} \cong \unit \). Equivalently, \(X\) is invertible if the functor \(X \otimes -\colon C \to C\) is an equivalence.
The full subanima \(\Pic (C) \subseteq C^{\simeq }\) spanned by the invertible objects is called the Picard anima of \(C\).
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