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  1. A vertex of a plane digraph is bimodal if all its incoming edges (and hence all its outgoing edges) are consecutive in the cyclic order around it. A plane digraph is bimodal if all its vertices are bimodal. Bimodality is at the heart of many types of graph layouts, such as upward drawings, level-planar drawings, and L-drawings. If the graph is not bimodal, the Maximum Bimodal Subgraph (MBS) problem asks for an embedding-preserving bimodal subgraph with the maximum number of edges. We initiate the study of the MBS problem from the parameterized complexity perspective with two main results: (i) we describe an FPT algorithm parameterized by the branchwidth (and hence by the treewidth) of the graph; (ii) we establish that MBS parameterized by the number of non-bimodal vertices admits a polynomial kernel. As the byproduct of these results, we obtain a subexponential FPT algorithm and an efficient polynomial-time approximation scheme for MBS. 
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    Free, publicly-accessible full text available January 10, 2025
  2. Intersection graphs of planar geometric objects such as intervals, disks, rectangles and pseudodisks are well-studied. Motivated by various applications, Butman et al. (ACM Trans. Algorithms, 2010) considered algorithmic questions in intersection graphs of $t$-intervals. A $t$-interval is a union of $t$ intervals --- these graphs are also referred to as multiple-interval graphs. Subsequent work by Kammer et al.\ (APPROX-RANDOM 2010) considered intersection graphs of $t$-disks (union of $t$ disks), and other geometric objects. In this paper we revisit some of these algorithmic questions via more recent developments in computational geometry. For the \emph{minimum-weight dominating set problem} in $t$-interval graphs, we obtain a polynomial-time $O(t \log t)$-approximation algorithm, improving upon the previously known polynomial-time $t^2$-approximation by Butman et al. In the same class of graphs we show that it is $\NP$-hard to obtain a $(t-1-\epsilon)$-approximation for any fixed $t \ge 3$ and $\epsilon > 0$. The approximation ratio for dominating set extends to the intersection graphs of a collection of $t$-pseudodisks (nicely intersecting $t$-tuples of closed Jordan domains). We obtain an $\Omega(1/t)$-approximation for the \emph{maximum-weight independent set} in the intersection graph of $t$-pseudodisks in polynomial time. Our results are obtained via simple reductions to existing algorithms by appropriately bounding the union complexity of the objects under consideration. 
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  3. Abstract

    We consider the problem of covering multiple submodular constraints. Given a finite ground setN, a weight function$$w: N \rightarrow \mathbb {R}_+$$w:NR+,rmonotone submodular functions$$f_1,f_2,\ldots ,f_r$$f1,f2,,froverNand requirements$$k_1,k_2,\ldots ,k_r$$k1,k2,,krthe goal is to find a minimum weight subset$$S \subseteq N$$SNsuch that$$f_i(S) \ge k_i$$fi(S)kifor$$1 \le i \le r$$1ir. We refer to this problem asMulti-Submod-Coverand it was recently considered by Har-Peled and Jones (Few cuts meet many point sets. CoRR.arxiv:abs1808.03260Har-Peled and Jones 2018) who were motivated by an application in geometry. Even with$$r=1$$r=1Multi-Submod-Covergeneralizes the well-known Submodular Set Cover problem (Submod-SC), and it can also be easily reduced toSubmod-SC. A simple greedy algorithm gives an$$O(\log (kr))$$O(log(kr))approximation where$$k = \sum _i k_i$$k=ikiand this ratio cannot be improved in the general case. In this paper, motivated by several concrete applications, we consider two ways to improve upon the approximation given by the greedy algorithm. First, we give a bicriteria approximation algorithm forMulti-Submod-Coverthat covers each constraint to within a factor of$$(1-1/e-\varepsilon )$$(1-1/e-ε)while incurring an approximation of$$O(\frac{1}{\epsilon }\log r)$$O(1ϵlogr)in the cost. Second, we consider the special case when each$$f_i$$fiis a obtained from a truncated coverage function and obtain an algorithm that generalizes previous work on partial set cover (Partial-SC), covering integer programs (CIPs) and multiple vertex cover constraints Bera et al. (Theoret Comput Sci 555:2–8 Bera et al. 2014). Both these algorithms are based on mathematical programming relaxations that avoid the limitations of the greedy algorithm. We demonstrate the implications of our algorithms and related ideas to several applications ranging from geometric covering problems to clustering with outliers. Our work highlights the utility of the high-level model and the lens of submodularity in addressing this class of covering problems.

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