Questions tagged [planar-graphs]

A planar graph is a graph (in the combinatorial sense) that can be embedded in a plane such that the edges only intersect at vertices. Consider tagging with [tag:combinatorics] and [tag:graph-theory]. For embeddings into higher-genus spaces, use [tag:graph-embeddings].

A planar graph is a graph (in the combinatorial sense) that can be embedded in a plane such that the edges only intersect at vertices. Consider tagging with and .

A finite graph $G$ is planar if and only if no minor of $G$ is the complete graph $K_5$, or the complete bipartite graph $K_{3,3}$ (Wagner's Theorem).

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Number of simple edge-disjoint paths needed to cover a planar graph

Let $G=(V,E)$ be a graph with $|E|=m$ of a graph class $\mathcal{G}$. A path-cover $\mathcal{P}=\{P_1,\ldots,P_k\}$ is a partition of $E$ into edge-disjoint simple paths. The size of the cover is $\sigma(\mathcal{P})=k$. I am interested in upper and…
A.Schulz
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Four color theorem disproof?

My brother in law and I were discussing the four color theorem; neither of us are huge math geeks, but we both like a challenge, and tonight we were discussing the four color theorem and if there were a way to disprove it. After some time scribbling…
Doktor J
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Euler's formula doesn't work for null graph?

Given the null graph with no edges or vertices, we have a connected planar graph as no edges cross when this graph is drawn in the plane, and the fact that any two distinct vertices have a path between them is vacuously true. However, Euler's…
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Is this graph a planar graph or not?

I've been trying to find out if this graph is planar or not for a while and have really been coming up short when it comes to creating a planar drawing of the graph. My intuition is telling me that it's non-planar, but I cannot find any subgraph of…
John21
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How to prove that a simple graph having 11 or more vertices or its complement is not planar?

It is an exercise on a book again. If a simple graph $G$ has $11$ or more vertices, then either $G$ or its complement $\overline { G } $ is not planar. How to begin with this? Induction? Thanks for your help!
tamlok
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Checking whether a graph is planar

I have to check whether a graph is planar. The given type is $$ e ≤ 3v − 6 .$$ From Wikipedia: Note that these theorems provide necessary conditions for planarity that are not sufficient conditions, and therefore can only be used to prove a…
GorillaApe
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Algorithms for "pleasing" drawings of planar graphs, possibly on sphere

What algorithms exist to draw planar graphs without edge crossings in a way that they are easy to interpret by humans? There are multiple algorithms that can handle any planar graph, such as Schnyder's algorithm or Chrobak-Payne. These typically…
Szabolcs
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Every planar graph has a vertex of degree at most 5.

I am trying to prove the following statement, any help!? Prove that every planar graph has a vertex of degree at most 5.
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"Planar" graphs on Möbius strips

Is there an easy way to tell if a graph can be embedded on a Möbius strip (with no edges crossing)? A specific version of this: if a simple graph with an odd number of vertices has all vertices of degree 4, can it be embedded on a Möbius…
Jack Schmidt
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A space curve is planar if and only if its torsion is everywhere 0

Can someone please explain this proof to me. I know that a circle is planar and has nonzero constant curvature, so this must be an exception, but I am a little lost on the proof. Thanks!
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Can there exist an uncountable planar graph?

I'm currently revising a course on graph theory that I took earlier this year. While thinking about planar graphs, I noticed that a finite planar graph corresponds to a (finite) polygonisation of the Euclidean plane (or whichever surface you're…
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Can you explain to me why this proof by induction is not flawed? (Domain is graph theory, but that is secondary)

Background I am following this MIT OCW course on mathematics for computer science. In one of the recitations they come to the below result: Official solution Task: A planar graph is a graph that can be drawn without any edges crossing. Also, any…
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5-color coloring game.

Let there be two players, $A$ and $B$, and a map. They now play a game such that: Player $A$ picks a region and player $B$ colors it such that the region is a different color than all adjacent regions. Player $B$ wins if at the end of the game,…
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Which graphs can be drawn using straight lines with no disjoint edges?

What is the class of graphs that can be drawn using only straight lines with no two edges disjoint? Edges are disjoint when they don't cross and they don't share a vertex. Vertices should be in general position (no three on a line). I know that for…
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Why is this proof for the four color theorem considered wrong?

I'd like to think I found a proof for the four color theorem, but I also know that it took far smarter people than me a computer simulation to prove. Still, I don't see why this logic should be flawed. If you'd explain to me plainly, I'd love it: 1-…
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