An
independent set
in a graph
G
is a set
S
of pairwise non-adjacent vertices in
G
. A family
\(\mathcal {F} \)
of independent sets in
G
is called a
k
-
independence covering family
if for every independent set
I
in
G
of size at most
k
, there exists an
\(S \in \mathcal {F} \)
such that
I
⊆
S
. Lokshtanov et al. [ACM Transactions on Algorithms, 2020] showed that graphs of degeneracy
d
admit
k
-independence covering families of size
\(\binom{k(d+1)}{k} \cdot 2^{o(kd)} \cdot \log n \)
, and used this result to design efficient parameterized algorithms for a number of problems, including
Stable Odd Cycle Transversal
and
Stable Multicut
.
In light of the results of Lokshtanov et al. [ACM Transactions on Algorithms, 2020] it is quite natural to ask whether even more general families of graphs admit
k
-independence covering families of size
f
(
k
)
n
O
(1)
. Graphs that exclude a complete bipartite graph
K
d
+ 1,
d
+ 1
with
d
+ 1 vertices on both sides as a subgraph, called
K
d
+ 1,
d
+ 1
-
free graphs
, are a frequently considered generalization of
d
-degenerate graphs. This motivates the question whether
K
d
,
d
-free graphs admit
k
-independence covering families of size
f
(
k
,
d
)
n
O
(1)
. Our main result is a resounding “no” to this question – specifically we prove that even
K
2, 2
-free graphs (or equivalently
C
4
-free graphs) do not admit
k
-independence covering families of size
\(f(k)n^{\frac{k}{4}-\epsilon } \)
.