I am reading Chapter 9 Approximation Algorithms of Dasgupts et al.'s Algorithm book for vertex cover approximation and they bring up the concept of matchings.

To support this, I am also watching Coursera's Approximation algorithms course by EIT digital.

This video mentions using disjoint edges and would like to confirm that the 2 are actually the same and I am not missing and subtle details.

Coursera's algorithm


1 Answer 1


Two disjoint edges $(a,b)$ and $(c,d)$ are edges that do not share any endpoint, i.e., $\{a,b\} \cap \{c,d\} = \emptyset$. A matching is a set of pairwise disjoint edges.

  • $\begingroup$ Still not clear. Are they different or the same? $\endgroup$ Apr 9, 2021 at 18:29
  • $\begingroup$ What does pairwise mean/add here? I see each edge as connecting a pair of vertices. $\endgroup$ Apr 9, 2021 at 18:29
  • $\begingroup$ Pairwise means that the condition holds for any distinct pair of edges in the matching. Formally, a set $M$ of edges is a matching if $\forall e_1 = (a,b) \in M, \forall e_2 = (c,d) \in M$, if $e_1 \neq e_2$ then $\{a,b\} \cap \{c,d\} = \emptyset$. $\endgroup$
    – Steven
    Apr 9, 2021 at 18:32
  • $\begingroup$ I am still unclear. The math text is what I don't get. Disjoint edges don't share any vertices but a matching is a set of edges such that each pair of edges picked from the set is disjoint? $\endgroup$ Apr 10, 2021 at 13:00
  • 1
    $\begingroup$ Yes. What's unclear about that? Being disjoint is a property about pairs of edges. A matching is a set in which each distinct pair of edges satisfies that property. $\endgroup$
    – Steven
    Apr 10, 2021 at 13:11

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