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Chords of longest cycles ★★★

Author(s): Thomassen

Conjecture   If $ G $ is a 3-connected graph, every longest cycle in $ G $ has a chord.

Keywords: chord; connectivity; cycle

Do any three longest paths in a connected graph have a vertex in common? ★★

Author(s): Gallai

Conjecture   Do any three longest paths in a connected graph have a vertex in common?


Chromatic number of $\frac{3}{3}$-power of graph ★★


Let $ G $ be a graph and $ m,n\in \mathbb{N} $. The graph $ G^{\frac{m}{n}} $ is defined to be the $ m $-power of the $ n $-subdivision of $ G $. In other words, $ G^{\frac{m}{n}}=(G^{\frac{1}{n}})^m $.

Conjecture   Let $ G $ be a graph with $ \Delta(G)\geq 2 $. Then $ \chi(G^{\frac{3}{3}})\leq 2\Delta(G)+1 $.


3-Edge-Coloring Conjecture ★★★

Author(s): Arthur; Hoffmann-Ostenhof

Conjecture   Suppose $ G $ with $ |V(G)|>2 $ is a connected cubic graph admitting a $ 3 $-edge coloring. Then there is an edge $ e \in E(G) $ such that the cubic graph homeomorphic to $ G-e $ has a $ 3 $-edge coloring.

Keywords: 3-edge coloring; 4-flow; removable edge

r-regular graphs are not uniquely hamiltonian. ★★★

Author(s): Sheehan

Conjecture   If $ G $ is a finite $ r $-regular graph, where $ r > 2 $, then $ G $ is not uniquely hamiltonian.

Keywords: hamiltonian; regular; uniquely hamiltonian

Partition of Complete Geometric Graph into Plane Trees ★★


Conjecture   Every complete geometric graph with an even number of vertices has a partition of its edge set into plane (i.e. non-crossing) spanning trees.

Keywords: complete geometric graph, edge colouring

Smooth 4-dimensional Poincare conjecture ★★★★

Author(s): Poincare; Smale; Stallings

Conjecture   If a $ 4 $-manifold has the homotopy type of the $ 4 $-sphere $ S^4 $, is it diffeomorphic to $ S^4 $?

Keywords: 4-manifold; poincare; sphere

Book Thickness of Subdivisions ★★

Author(s): Blankenship; Oporowski

Let $ G $ be a finite undirected simple graph.

A $ k $-page book embedding of $ G $ consists of a linear order $ \preceq $ of $ V(G) $ and a (non-proper) $ k $-colouring of $ E(G) $ such that edges with the same colour do not cross with respect to $ \preceq $. That is, if $ v\prec x\prec w\prec y $ for some edges $ vw,xy\in E(G) $, then $ vw $ and $ xy $ receive distinct colours.

One can think that the vertices are placed along the spine of a book, and the edges are drawn without crossings on the pages of the book.

The book thickness of $ G $, denoted by bt$ (G) $ is the minimum integer $ k $ for which there is a $ k $-page book embedding of $ G $.

Let $ G' $ be the graph obtained by subdividing each edge of $ G $ exactly once.

Conjecture   There is a function $ f $ such that for every graph $ G $, $$   \text{bt}(G) \leq f( \text{bt}(G') )\enspace.   $$

Keywords: book embedding; book thickness

Primitive pythagorean n-tuple tree ★★


Conjecture   Find linear transformation construction of primitive pythagorean n-tuple tree!


Jacobian Conjecture ★★★

Author(s): Keller

Conjecture   Let $ k $ be a field of characteristic zero. A collection $ f_1,\ldots,f_n $ of polynomials in variables $ x_1,\ldots,x_n $ defines an automorphism of $ k^n $ if and only if the Jacobian matrix is a nonzero constant.

Keywords: Affine Geometry; Automorphisms; Polynomials

Inscribed Square Problem ★★

Author(s): Toeplitz

Conjecture   Does every Jordan curve have 4 points on it which form the vertices of a square?

Keywords: simple closed curve; square

Complete bipartite subgraphs of perfect graphs ★★

Author(s): Fox

Problem   Let $ G $ be a perfect graph on $ n $ vertices. Is it true that either $ G $ or $ \bar{G} $ contains a complete bipartite subgraph with bipartition $ (A,B) $ so that $ |A|, |B| \ge n^{1 - o(1)} $?

Keywords: perfect graph

Transversal achievement game on a square grid ★★

Author(s): Erickson

Problem   Two players alternately write O's (first player) and X's (second player) in the unoccupied cells of an $ n \times  n $ grid. The first player (if any) to occupy a set of $ n $ cells having no two cells in the same row or column is the winner. What is the outcome of the game given optimal play?

Keywords: game

Graceful Tree Conjecture ★★★


Conjecture   All trees are graceful

Keywords: combinatorics; graceful labeling

Extremal problem on the number of tree endomorphism ★★

Author(s): Zhicong Lin

Conjecture   An endomorphism of a graph is a mapping on the vertex set of the graph which preserves edges. Among all the $ n $ vertices' trees, the star with $ n $ vertices has the most endomorphisms, while the path with $ n $ vertices has the least endomorphisms.


3-Colourability of Arrangements of Great Circles ★★

Author(s): Felsner; Hurtado; Noy; Streinu

Consider a set $ S $ of great circles on a sphere with no three circles meeting at a point. The arrangement graph of $ S $ has a vertex for each intersection point, and an edge for each arc directly connecting two intersection points. So this arrangement graph is 4-regular and planar.

Conjecture   Every arrangement graph of a set of great circles is $ 3 $-colourable.

Keywords: arrangement graph; graph coloring

KPZ Universality Conjecture ★★★


Conjecture   Formulate a central limit theorem for the KPZ universality class.

Keywords: KPZ equation, central limit theorem

Friendly partitions ★★

Author(s): DeVos

A friendly partition of a graph is a partition of the vertices into two sets so that every vertex has at least as many neighbours in its own class as in the other.

Problem   Is it true that for every $ r $, all but finitely many $ r $-regular graphs have friendly partitions?

Keywords: edge-cut; partition; regular

Finite entailment of Positive Horn logic ★★

Author(s): Martin

Question   Positive Horn logic (pH) is the fragment of FO involving exactly $ \exists, \forall, \wedge, = $. Does the fragment $ pH \wedge \neg pH $ have the finite model property?

Keywords: entailment; finite satisfiability; horn logic

Triangle free strongly regular graphs ★★★


Problem   Is there an eighth triangle free strongly regular graph?

Keywords: strongly regular; triangle free