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Given

$ R_1 = \{(1,2),(5,3)\}\quad\quad R_2 = \{(6,4),(5,7)\}$

What is $R_2 \circ R_1$?

Because in my understanding, using the example

$ R_3 = \{(1,2),(3,4)\} \quad\quad R_4 = \{ (2,5),(6,7)\}$

Then $R_4\circ R_3 = \{(1,5)\}.\;$ Am I correct?

So given that case, if $R_2 \circ R_1 = (a,b),\;a$ comes from $R_1$, right? While $b$ comes from $R_2$?

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We have by definition (assembled from your example) \[ R_2 \circ R_1 = \{(a,b) \mid \exists c : (a,c) \in R_1, (c,b) \in R_2\} \] In the $R_4 \circ R_3$ example $(1,5) \in R_4 \circ R_3$ as we can "plug in" $2$ in the middle where $(1,2) \in R_3$, $(2,5) \in R_4$.

If there is no "matching" pair from $R_1$ and $R_2$ as it is in your case, there is no $(a,b)$ fulfilling the condition for being a member of $R_2 \circ R_1$, we have by definition $R_2 \circ R_1 = \emptyset$.

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