# Recent Activity

## Arc-disjoint out-branching and in-branching ★★

Author(s): Thomassen

**Conjecture**There exists an integer such that every -arc-strong digraph with specified vertices and contains an out-branching rooted at and an in-branching rooted at which are arc-disjoint.

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## Strong edge colouring conjecture ★★

A strong edge-colouring of a graph is a edge-colouring in which every colour class is an induced matching; that is, any two vertices belonging to distinct edges with the same colour are not adjacent. The strong chromatic index is the minimum number of colours in a strong edge-colouring of .

**Conjecture**

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## Long directed cycles in diregular digraphs ★★★

Author(s): Jackson

**Conjecture**Every strong oriented graph in which each vertex has indegree and outdegree at least contains a directed cycle of length at least .

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## Splitting a digraph with minimum outdegree constraints ★★★

Author(s): Alon

**Problem**Is there a minimum integer such that the vertices of any digraph with minimum outdegree can be partitioned into two classes so that the minimum outdegree of the subgraph induced by each class is at least ?

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## Stable set meeting all longest directed paths. ★★

Author(s): Laborde; Payan; Xuong N.H.

**Conjecture**Every digraph has a stable set meeting all longest directed paths

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## Ádám's Conjecture ★★★

Author(s): Ádám

**Conjecture**Every digraph with at least one directed cycle has an arc whose reversal reduces the number of directed cycles.

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## Caccetta-Häggkvist Conjecture ★★★★

Author(s): Caccetta; Häggkvist

**Conjecture**Every simple digraph of order with minimum outdegree at least has a cycle with length at most

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## Directed path of length twice the minimum outdegree ★★★

Author(s): Thomassé

**Conjecture**Every oriented graph with minimum outdegree contains a directed path of length .

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## Antidirected trees in digraphs ★★

Author(s): Addario-Berry; Havet; Linhares Sales; Reed; Thomassé

An antidirected tree is an orientation of a tree in which every vertex has either indegree 0 or outdergree 0.

**Conjecture**Let be a digraph. If , then contains every antidirected tree of order .

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## Decomposing an even tournament in directed paths. ★★★

Author(s): Alspach; Mason; Pullman

**Conjecture**Every tournament on an even number of vertices can be decomposed into directed paths.

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## Oriented trees in n-chromatic digraphs ★★★

Author(s): Burr

**Conjecture**Every digraph with chromatic number at least contains every oriented tree of order as a subdigraph.

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## Discrete Logarithm Problem ★★★

Author(s):

If is prime and , we write if satisfies . The problem of finding such an integer for a given (with ) is the *Discrete Log Problem*.

**Conjecture**There does not exist a polynomial time algorithm to solve the Discrete Log Problem.

Keywords: discrete log; NP

## Good Edge Labelings ★★

Author(s): Araújo; Cohen; Giroire; Havet

**Question**What is the maximum edge density of a graph which has a good edge labeling?

We say that a graph is *good-edge-labeling critical*, if it has no good edge labeling, but every proper subgraph has a good edge labeling.

**Conjecture**For every , there is only a finite number of good-edge-labeling critical graphs with average degree less than .

Keywords: good edge labeling, edge labeling

## 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 grid. The first player (if any) to occupy a set of 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

## Special Primes ★

Author(s): George BALAN

**Conjecture**Let be a prime natural number. Find all primes , such that .

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## Three-chromatic (0,2)-graphs ★★

Author(s): Payan

**Question**Are there any (0,2)-graphs with chromatic number exactly three?

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## Choice Number of k-Chromatic Graphs of Bounded Order ★★

Author(s): Noel

**Conjecture**If is a -chromatic graph on at most vertices, then .

Keywords: choosability; complete multipartite graph; list coloring

## The Riemann Hypothesis ★★★★

Author(s): Riemann

The zeroes of the Riemann zeta function that are inside the Critical Strip (i.e. the vertical strip of the complex plane where the real part of the complex variable is in ]0;1[), are actually located on the Critical line ( the vertical line of the complex plane with real part equal to 1/2)

Keywords: Millenium Problems; zeta

## Euler-Mascheroni constant ★★★

Author(s):

**Question**Is Euler-Mascheroni constant an transcendental number?

Keywords: constant; Euler; irrational; Mascheroni; rational; transcendental

## Graham's conjecture on tree reconstruction ★★

Author(s): Graham

**Problem**for every graph , we let denote the line graph of . Given that is a tree, can we determine it from the integer sequence ?

Keywords: reconstruction; tree