• Characterisation of Relations Admitting Left Kan Extensions Along Them. The following conditions are equivalent:
    1. The left Kan extension
      \[ \text{Lan}_{R}\colon \mathbf{Rel}\webleft (A,X\webright )\to \mathbf{Rel}\webleft (B,X\webright ) \]

      along $R$ exists.

    2. The relation $R$ admits a left adjoint in $\textbf{Rel}$.
    3. The relation $R$ is of the form $f^{-1}$ (as in Definition 7.3.2.1.1) for some function $f$.

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