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The Hitting Set problem, when parameterized by the size $k$ of the hitting set, is W[2]-hard. Is it also W[2]-hard when parameterized by $k$ plus the number of subsets in the instance?

I explain in a bit more detail. A Hitting Set instance consists of a universe $U = \{ u_1, \dots, u_n\}$ and a set $S = \{ S_1, \dots, S_m\} \subseteq \mathcal{P}(U)$ together with a natural number $k$. A hitting set is a set $H \subseteq U$ of size $k$ such that for each $i \in [m]$, $H \cap S_i \neq \emptyset$. We know that Hitting Set parameteized by $k$ is W[2]-complete. Is it still W[2]-hard when parameterized by $k + |S|?$

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This is FPT, because by interchanging sets with elements in the usual way, this is just set cover where the universe size $n$ is the parameter. This is known to be FPT (see e.g. the parameterized algorithms book, Chapter 6)

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  • $\begingroup$ Related to this: do we know some bound on the minimal number of sets over all equivalent Hitting Set instances? That is, if I have an instance $(U, S, k)$ of Hitting Set with $n$ elements in the universe and $m = |S|$ sets and I find an equivalent instance $(U, S_0, k)$ (i.e. they have the same hitting sets) such that $|S_0|$ is minimal over all equivalent instances... can we say something about $|S_0|$? Is it perhaps bounded by some function of $k$? I would imagine that if we parameterize by $|S_0|$ instead of by $|S|$ then the problem becomes hard? $\endgroup$ Jun 4 at 16:12

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