Demonic composition

In mathematics, demonic composition is an operation on binary relations that is similar to the ordinary composition of relations but is robust to refinement of the relations into (partial) functions or injective relations.

Unlike ordinary composition of relations, demonic composition is not associative.

Definition

Suppose $R$ is a binary relation between $X$ and $Y$ and $S$ is a relation between $Y$ and $Z.$ Their Template:Visible anchor $R{\mathbf{\text{<mo>;</mo>}}}^{\to }S$ is a relation between $X$ and $Z.$ Its graph is defined as $$\{(x,z):x(S\circ R)z\text{ and for all }y\in Y(xRy\text{ implies }ySz)\}.$$

Conversely, their Template:Visible anchor $R{\mathbf{\text{<mo>;</mo>}}}^{\leftarrow }S$ is defined by $$\{(x,z):x(S\circ R)z\text{ and for all }y\in Y(ySz\text{ implies }xRy)\}.$$

References

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