Consider the following variant of set cover:

Given: Target set $T$ and a collection of sets $\mathcal{C}$, such that $T \subseteq \bigcup_{C \in \mathcal{C}} C$.

Wanted: A subset $\mathcal{C'}$ of $\mathcal{C}$, such that $T \subseteq \bigcup_{C \in \cal{C'}} C$ and $|\bigcup_{C \in \cal{C'}} C|$ is minimal.

In other words, we are looking for a covering of $T$ that covers as few additional elements as possible.

It is relatively easy to see that this problem is NP-complete.

Is this a known problem? What is its name?

  • $\begingroup$ One of the standard generalizations of Set Cover is minimizing a linear function subject to a submodular constraint. See e.g. algnotes.info/on/background/set-cover-wolsey . Doesn't look like it quite captures your problem though. $\endgroup$
    – Neal Young
    Nov 8, 2023 at 20:42
  • $\begingroup$ Ouch, indeed the $\sum$'s should be $\bigcup$'s. $\endgroup$ Nov 8, 2023 at 21:29

1 Answer 1


We can abstract this problem as submodular minimization (minimize additional covers) under submodular cover (set cover) constraint. I'll point you towards two papers in this area.

The first paper proves that a primal-dual algorithm achieves constant approximation if maximum frequency is fixed. The second paper gives a more careful bound.

  • $\begingroup$ The Iwata and Nagano paper proves a strong lower bound on the approximation when the maximum frequency is not bounded. The lower bound should probably hold for the OPs problem as well. $\endgroup$ Dec 4, 2023 at 0:46

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