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Search options questions only not deleted user 156936

Questions about the branch of combinatorics called graph theory (not to be used for questions concerning the graph of a function). This tag can be further specialized via using it in combination with more specialized tags such as extremal-graph-theory, spectral-graph-theory, algebraic-graph-theory, topological-graph-theory, random-graphs, graph-colorings and several others.

77 votes
4 answers
15k views

What are good mathematical models for spider webs?

Sometimes I see spider webs in very complex surroundings, like in the middle of twigs in a tree or in a bush. I keep thinking “if you understand the spider web, you understand the space around it”. W …
Claus's user avatar
  • 6,937
74 votes
29 answers
8k views

Proofs where higher dimension or cardinality actually enabled much simpler proof?

I am very interested in proofs that become shorter and simpler by going to higher dimension in $\mathbb R^n$, or higher cardinality. By "higher" I mean that the proof is using higher dimension or card …
35 votes
4 answers
4k views

An intelligent ant living on a torus or sphere – Does it have a universal way to find out?

I wanted to ask a question about topological invariants and whether they are connected in a fundamental or universal way. I am not an expert in topology, so please let me ask this question by way of a …
Claus's user avatar
  • 6,937
25 votes
3 answers
1k views

Algebraic graph invariant $\mu(G)$ which links Four-Color-Theorem with Schrödinger operators...

30 years ago, Yves Colin de Verdière introduced the algebraic graph invariant $\mu(G)$ for any undirected graph $G$, see [1]. It was motivated by the study of the second eigenvalue of certain Schrödin …
Claus's user avatar
  • 6,937
18 votes
1 answer
1k views

Sperner's Lemma implies Tucker's Lemma - simple combinatorial proof

Sperner’s Lemma is often called the "combinatorial analog" of Brouwer’s Fixed Point Theorem, and similarly Tucker’s Lemma is often called the combinatorial analog of Borsuk–Ulam’s Theorem. We can fa …
Claus's user avatar
  • 6,937
17 votes
1 answer
651 views

Graph embeddings in the projective plane: for the 35 forbidden minors, do we know their Coli...

The Graph Minor Theorem of Robertson and Seymour asserts that any minor-closed graph property is determined by a finite set of forbidden graph minors. It is a broad generalization e.g. of the Kuratows …
Claus's user avatar
  • 6,937
11 votes
1 answer
368 views

Graph with path of length $\geq n$ along grid diagonals - a known result in graph theory?

Is the following lemma a well known result in graph theory? I am studying a basic existence result that appears to be simple yet powerful. I have not seen it stated as an important result in graph …
Claus's user avatar
  • 6,937
3 votes
1 answer
155 views

Structure of boundary labelling in Sperner‘s Lemma

Consider a triangulated polygon in the 2-dimensional plane, where each vertex is labelled green, blue, or orange. Sperner's Lemma asserts that a fully-colored triangle exists in the triangulation, if …
Claus's user avatar
  • 6,937