10 questions linked to/from NP-hard problems on trees
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### NP-hard problems on paths

everybody knows there exist many decision problems which are NP-hard on general graphs, but I'm interested in problems that are even NP-hard when the underlying graph is a path. So, can you help me to ...
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### P-complete problems on trees

This question is related to one of my previous questions, NP-hard problems on trees. I am looking for problems that are P-complete on trees.
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### Directed NP-hard problems on DAGs

Tree width measures how close a graph is to a tree. Several NP-hard problems are tractable on graphs with bounded tree width. If a problem remains NP-hard on trees then tree width cannot save us. This ...
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### In-approximability results in severely restricted graph classes

Longest path problem is not polynomial-time approximable to any constant factor in cubic Hamiltonian graphs (Longest path $\notin APX$ unless $P=NP$). I don't know if it remains in-approximable in ...
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### What separates easy global problems from hard global problems on graphs of bounded treewidth?

Plenty of hard graph problems are solvable in polynomial time on graphs of bounded treewidth. Indeed, textbooks typically use e.g. independet set as an example, which is a local problem. Roughly, a ...
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### NP-hard problems on cographs

This question is similar to NP-hard problems on trees: There is a large number of NP-complete problems that are tractable on cographs. Are there any known problems that remain NP-complete when ...
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### Measuring the connectedness of a graph, and applying it to NP problems

I'm looking for a way to measure how interconnected a graph is. It's well known that graphs can be broken down into connected components. It seems, though, that even in the cases where the graph is ...
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### NP-hard problems on the class of caterpillars

My question is whether there exist an NP-hard problem that has only a caterpillar as input. By saying only caterpillar as input, I wanted to emphasize that no function (eg: weights on vertices or ...
This question was motivated by a question asked on stackoverflow. Suppose you are given a rooted tree $T$ (i.e. there is a root and nodes have children etc) on $n$ nodes (labelled $1, 2, \dots, n$). ...