Parameterized Lower Bounds for Problems in P via Fine-Grained Cross-Compositions

We provide a general framework to exclude parameterized running times of the form $O(\ell^β+ n^γ)$ for problems that have polynomial running time lower bounds under hypotheses from fine-grained complexity. Our framework is based on cross-compositions from parameterized complexity. We (conditionally) exclude running times of the form $O(\ell^{γ/{(γ-1)} - ε} + n^γ)$ for any $1<γ<2$ and $ε>0$ for the following problems: - Longest Common Subsequence: Given two length-$n$ strings and $\ell\in\mathbb{N}$, is there a common subsequence of length $\ell$? - Discrete Fréchet Distance: Given two lists of $n$ points each and $k\in \mathbb{N}$, is the Fréchet distance of the lists at most $k$? Here $\ell$ is the maximum number of points which one list is ahead of the other list in an optimum traversal. Moreover, we exclude running times $O(\ell^{{2γ}/{(γ-1)}-ε} + n^γ)$ for any $1<γ<3$ and $ε>0$ for: - Negative Triangle: Given an edge-weighted graph with $n$ vertices, is there a triangle whose sum of edge-weights is negative? Here $\ell$ is the order of a maximum connected component. - Triangle Collection: Given a vertex-colored graph with $n$ vertices, is there for each triple of colors a triangle whose vertices have these three colors? Here $\ell$ is the order of a maximum connected component. - 2nd Shortest Path: Given an $n$-vertex edge-weighted directed graph, two vertices $s$ and $t$, and $k \in \mathbb{N}$, has the second longest $s$-$t$-path length at most $k$? Here $\ell$ is the directed feedback vertex set. Except for 2nd Shortest Path all these running time bounds are tight, that is, algorithms with running time $O(\ell^{γ/{(γ-1)}} + n^γ)$ for any $1 < γ< 2$ and $O(\ell^{{2γ}/{(γ-1)}} + n^γ)$ for any $1 < γ< 3$, respectively, are known.

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