# Recent Activity

## Consecutive non-orientable embedding obstructions ★★★

Author(s):

**Conjecture**Is there a graph that is a minor-minimal obstruction for two non-orientable surfaces?

## Diagonal Ramsey numbers ★★★★

Author(s): Erdos

Let denote the diagonal Ramsey number.

**Conjecture**exists.

**Problem**Determine the limit in the above conjecture (assuming it exists).

Keywords: Ramsey number

## The 4x5 chessboard complex is the complement of a link, which link? ★★

Author(s): David Eppstein

**Problem**Ian Agol and Matthias Goerner observed that the 4x5 chessboard complex is the complement of many distinct links in the 3-sphere. Their observation is non-constructive, as it uses the resolution of the Poincare Conjecture. Find specific links that have the 4x5 chessboard complex as their complement.

Keywords: knot theory, links, chessboard complex

## Elementary symmetric of a sum of matrices ★★★

Author(s):

**Problem**

Given a Matrix , the -th elementary symmetric function of , namely , is defined as the sum of all -by- principal minors.

Find a closed expression for the -th elementary symmetric function of a sum of N -by- matrices, with by using partitions.

Keywords:

## Monochromatic empty triangles ★★★

Author(s):

If is a finite set of points which is 2-colored, an *empty triangle* is a set with so that the convex hull of is disjoint from . We say that is *monochromatic* if all points in are the same color.

**Conjecture**There exists a fixed constant with the following property. If is a set of points in general position which is 2-colored, then it has monochromatic empty triangles.

Keywords: empty triangle; general position; ramsey theory

## Edge-antipodal colorings of cubes ★★

Author(s): Norine

We let denote the -dimensional cube graph. A map is called *edge-antipodal* if whenever are antipodal edges.

**Conjecture**If and is edge-antipodal, then there exist a pair of antipodal vertices which are joined by a monochromatic path.

Keywords: antipodal; cube; edge-coloring

## Exponential Algorithms for Knapsack ★★

Author(s): Lipton

**Conjecture**

The famous 0-1 Knapsack problem is: Given and integers, determine whether or not there are values so that

The best known worst-case algorithm runs in time times a polynomial in . Is there an algorithm that runs in time ?

Keywords: Algorithm construction; Exponential-time algorithm; Knapsack

## Unsolvability of word problem for 2-knot complements ★★★

Author(s): Gordon

**Problem**Does there exist a smooth/PL embedding of in such that the fundamental group of the complement has an unsolvable word problem?

Keywords: 2-knot; Computational Complexity; knot theory

## Algorithm for graph homomorphisms ★★

Author(s): Fomin; Heggernes; Kratsch

**Question**

Is there an algorithm that decides, for input graphs and , whether there exists a homomorphism from to in time for some constant ?

Keywords: algorithm; Exponential-time algorithm; homomorphism

## Exact colorings of graphs ★★

Author(s): Erickson

**Conjecture**For , let be the statement that given any exact -coloring of the edges of a complete countably infinite graph (that is, a coloring with colors all of which must be used at least once), there exists an exactly -colored countably infinite complete subgraph. Then is true if and only if , , or .

Keywords: graph coloring; ramsey theory

## Graceful Tree Conjecture ★★★

Author(s):

**Conjecture**All trees are graceful

Keywords: combinatorics; graceful labeling

## Dividing up the unrestricted partitions ★★

Begin with the generating function for unrestricted partitions:

(1+x+x^2+...)(1+x^2+x^4+...)(1+x^3+x^6+...)...

Now change some of the plus signs to minus signs. The resulting series will have coefficients congruent, mod 2, to the coefficients of the generating series for unrestricted partitions. I conjecture that the signs may be chosen such that all the coefficients of the series are either 1, -1, or zero.

Keywords: congruence properties; partition

## The stubborn list partition problem ★★

Author(s): Cameron; Eschen; Hoang; Sritharan

**Problem**Does there exist a polynomial time algorithm which takes as input a graph and for every vertex a subset of , and decides if there exists a partition of into so that only if and so that are independent, is a clique, and there are no edges between and ?

Keywords: list partition; polynomial algorithm

## Long rainbow arithmetic progressions ★★

Author(s): Fox; Jungic; Mahdian; Nesetril; Radoicic

For let denote the minimal number such that there is a rainbow in every equinumerous -coloring of for every

**Conjecture**For all , .

Keywords: arithmetic progression; rainbow

## Reconstruction conjecture ★★★★

The *deck* of a graph is the multiset consisting of all unlabelled subgraphs obtained from by deleting a vertex in all possible ways (counted according to multiplicity).

**Conjecture**If two graphs on vertices have the same deck, then they are isomorphic.

Keywords: reconstruction

## Finding k-edge-outerplanar graph embeddings ★★

Author(s): Bentz

**Conjecture**It has been shown that a -outerplanar embedding for which is minimal can be found in polynomial time. Does a similar result hold for -edge-outerplanar graphs?

Keywords: planar graph; polynomial algorithm

## Approximation ratio for k-outerplanar graphs ★★

Author(s): Bentz

**Conjecture**Is the approximation ratio for the

*Maximum Edge Disjoint Paths*(MaxEDP) or the

*Maximum Integer Multiflow*problem (MaxIMF) bounded by a constant in -outerplanar graphs or tree-width graphs?

Keywords: approximation algorithms; planar graph; polynomial algorithm

## Approximation Ratio for Maximum Edge Disjoint Paths problem ★★

Author(s): Bentz

**Conjecture**Can the approximation ratio be improved for the Maximum Edge Disjoint Paths problem (MaxEDP) in planar graphs or can an inapproximability result stronger than -hardness?

Keywords: approximation algorithms; Disjoint paths; planar graph; polynomial algorithm

## Beneš Conjecture (graph-theoretic form) ★★★

Author(s): Beneš

**Problem ()**Find a sufficient condition for a straight -stage graph to be rearrangeable. In particular, what about a straight uniform graph?

**Conjecture ()**Let be a simple regular ordered -stage graph. Suppose that the graph is externally connected, for some . Then the graph is rearrangeable.

Keywords:

## Seymour's self-minor conjecture ★★★

Author(s): Seymour

**Conjecture**Every infinite graph is a proper minor of itself.

Keywords: infinite graph; minor