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Number Theory » Combinatorial N.T.

The 3n+1 conjecture ★★★

Author(s): Collatz

Conjecture   Let $ f(n) = 3n+1 $ if $ n $ is odd and $ \frac{n}{2} $ if $ n $ is even. Let $ f(1) = 1 $. Assume we start with some number $ n $ and repeatedly take the $ f $ of the current number. Prove that no matter what the initial number is we eventually reach $ 1 $.

Keywords: integer sequence

Posted by dododododo
updated July 11th, 2014
4 comments
Graph Theory » Coloring » Vertex coloring

List Hadwiger Conjecture ★★

Author(s): Kawarabayashi; Mohar

Conjecture   Every $ K_t $-minor-free graph is $ c t $-list-colourable for some constant $ c\geq1 $.

Keywords: Hadwiger conjecture; list colouring; minors

Posted by David Wood
updated July 7th, 2014
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Number Theory

Lucas Numbers Modulo m ★★

Author(s):

Conjecture   The sequence {L(n) mod m}, where L(n) are the Lucas numbers, contains a complete residue system modulo m if and only if m is one of the following: 2, 4, 6, 7, 14, 3^k, k >=1.

Keywords: Lucas numbers

Posted by Martin Erickson
updated July 1st, 2014
1 comment
Number Theory » Combinatorial N.T.

Divisibility of central binomial coefficients ★★

Author(s): Graham

Problem  (1)   Prove that there exist infinitely many positive integers $ n $ such that $$\gcd({2n\choose n}, 3\cdot 5\cdot 7) = 1.$$
Problem  (2)   Prove that there exists only a finite number of positive integers $ n $ such that $$\gcd({2n\choose n}, 3\cdot 5\cdot 7\cdot 11) = 1.$$

Keywords:

Posted by maxal
updated June 29th, 2014
5 comments
Graph Theory » Hypergraphs

¿Are critical k-forests tight? ★★

Author(s): Strausz

Conjecture  

Let $ H $ be a $ k $-uniform hypergraph. If $ H $ is a critical $ k $-forest, then it is a $ k $-tree.

Keywords: heterochromatic number

Posted by Dino
updated May 5th, 2014
1 comment
Combinatorics » Posets

Saturated $k$-Sperner Systems of Minimum Size ★★

Author(s): Morrison; Noel; Scott

Question   Does there exist a constant $ c>1/2 $ and a function $ n_0(k) $ such that if $ |X|\geq n_0(k) $, then every saturated $ k $-Sperner system $ \mathcal{F}\subseteq \mathcal{P}(X) $ has cardinality at least $ 2^{(1+o(1))ck} $?

Keywords: antichain; extremal combinatorics; minimum saturation; saturation; Sperner system

Posted by Jon Noel
updated April 12th, 2014
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Graph Theory » Coloring » Vertex coloring

List Colourings of Complete Multipartite Graphs with 2 Big Parts ★★

Author(s): Allagan

Question   Given $ a,b\geq2 $, what is the smallest integer $ t\geq0 $ such that $ \chi_\ell(K_{a,b}+K_t)= \chi(K_{a,b}+K_t) $?

Keywords: complete bipartite graph; complete multipartite graph; list coloring

Posted by Jon Noel
updated April 12th, 2014
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Geometry

Generalised Empty Hexagon Conjecture ★★

Author(s): Wood

Conjecture   For each $ \ell\geq3 $ there is an integer $ f(\ell) $ such that every set of at least $ f(\ell) $ points in the plane contains $ \ell $ collinear points or an empty hexagon.

Keywords: empty hexagon

Posted by David Wood
updated March 26th, 2014
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Geometry

General position subsets ★★

Author(s): Gowers

Question   What is the least integer $ f(n) $ such that every set of at least $ f(n) $ points in the plane contains $ n $ collinear points or a subset of $ n $ points in general position (no three collinear)?

Keywords: general position subset, no-three-in-line problem

Posted by David Wood
updated March 24th, 2014
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Graph Theory » Basic G.T. » Minors

Forcing a 2-regular minor ★★

Author(s): Reed; Wood

Conjecture   Every graph with average degree at least $ \frac{4}{3}t-2 $ contains every 2-regular graph on $ t $ vertices as a minor.

Keywords: minors

Posted by David Wood
updated March 16th, 2014
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Graph Theory

Fractional Hadwiger ★★

Author(s): Harvey; Reed; Seymour; Wood

Conjecture   For every graph $ G $,
(a) $ \chi_f(G)\leq\text{had}(G) $
(b) $ \chi(G)\leq\text{had}_f(G) $
(c) $ \chi_f(G)\leq\text{had}_f(G) $.

Keywords: fractional coloring, minors

Posted by David Wood
updated March 16th, 2014
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Topology

Generalized path-connectedness in proximity spaces ★★

Author(s): Porton

Let $ \delta $ be a proximity.

A set $ A $ is connected regarding $ \delta $ iff $ \forall X,Y \in \mathscr{P} A \setminus \{ \emptyset \} : \left( X \cup Y = A \Rightarrow X \mathrel{\delta} Y \right) $.

Conjecture   The following statements are equivalent for every endofuncoid $ \mu $ and a set $ U $:
    \item $ U $ is connected regarding $ \mu $. \item For every $ a, b \in U $ there exists a totally ordered set $ P \subseteq U $ such that $ \min P = a $, $ \max P = b $, and for every partion $ \{ X, Y \} $ of $ P $ into two sets $ X $, $ Y $ such that $ \forall x \in X, y \in Y : x < y $, we have $ X \mathrel{[ \mu]^{\ast}} Y $.

Keywords: connected; connectedness; proximity space

Posted by porton
updated February 26th, 2014
1 comment
Topology

Direct proof of a theorem about compact funcoids ★★

Author(s): Porton

Conjecture   Let $ f $ is a $ T_1 $-separable (the same as $ T_2 $ for symmetric transitive) compact funcoid and $ g $ is a uniform space (reflexive, symmetric, and transitive endoreloid) such that $ ( \mathsf{\tmop{FCD}}) g = f $. Then $ g = \langle f \times f \rangle^{\ast} \Delta $.

The main purpose here is to find a direct proof of this conjecture. It seems that this conjecture can be derived from the well known theorem about existence of exactly one uniformity on a compact set. But that would be what I call an indirect proof, we need a direct proof instead.

The direct proof may be constructed by correcting all errors an omissions in this draft article.

Direct proof could be better because with it we would get a little more general statement like this:

Conjecture   Let $ f $ be a $ T_1 $-separable compact reflexive symmetric funcoid and $ g $ be a reloid such that
    \item $ ( \mathsf{\tmop{FCD}}) g = f $; \item $ g \circ g^{- 1} \sqsubseteq g $.

Then $ g = \langle f \times f \rangle^{\ast} \Delta $.

Keywords: compact space; compact topology; funcoid; reloid; uniform space; uniformity

Posted by porton
updated February 8th, 2014
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Geometry

Dirac's Conjecture ★★

Author(s): Dirac

Conjecture   For every set $ P $ of $ n $ points in the plane, not all collinear, there is a point in $ P $ contained in at least $ \frac{n}{2}-c $ lines determined by $ P $, for some constant $ c $.

Keywords: point set

Posted by David Wood
updated January 22nd, 2014
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Combinatorics

Roller Coaster permutations ★★★

Author(s): Ahmed; Snevily

Let $ S_n $ denote the set of all permutations of $ [n]=\set{1,2,\ldots,n} $. Let $ i(\pi) $ and $ d(\pi) $ denote respectively the number of increasing and the number of decreasing sequences of contiguous numbers in $ \pi $. Let $ X(\pi) $ denote the set of subsequences of $ \pi $ with length at least three. Let $ t(\pi) $ denote $ \sum_{\tau\in X(\pi)}(i(\tau)+d(\tau)) $.

A permutation $ \pi\in S_n $ is called a Roller Coaster permutation if $ t(\pi)=\max_{\tau\in S_n}t(\tau) $. Let $ RC(n) $ be the set of all Roller Coaster permutations in $ S_n $.

Conjecture   For $ n\geq 3 $,
    \item If $ n=2k $, then $ |RC(n)|=4 $. \item If $ n=2k+1 $, then $ |RC(n)|=2^j $ with $ j\leq k+1 $.
Conjecture  (Odd Sum conjecture)   Given $ \pi\in RC(n) $,
    \item If $ n=2k+1 $, then $ \pi_j+\pi_{n-j+1} $ is odd for $ 1\leq j\leq k $. \item If $ n=2k $, then $ \pi_j + \pi_{n-j+1} = 2k+1 $ for all $ 1\leq j\leq k $.

Keywords:

Posted by Tanbir Ahmed
updated October 14th, 2013
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Algebra

Graphs of exact colorings ★★

Author(s):

Conjecture For $ c \geq m \geq 1 $, let $ P(c,m) $ be the statement that given any exact $ c $-coloring of the edges of a complete countably infinite graph (that is, a coloring with $ c $ colors all of which must be used at least once), there exists an exactly $ m $-colored countably infinite complete subgraph. Then $ P(c,m) $ is true if and only if $ m=1 $, $ m=2 $, or $ c=m $.

Keywords:

Posted by sabisood
updated October 3rd, 2013
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Graph Theory

Imbalance conjecture ★★

Author(s): Kozerenko

Conjecture   Suppose that for all edges $ e\in E(G) $ we have $ imb(e)>0 $. Then $ M_{G} $ is graphic.

Keywords: edge imbalance; graphic sequences

Posted by Sergiy Kozerenko
updated September 24th, 2013
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Topology

Every metamonovalued reloid is monovalued ★★

Author(s): Porton

Conjecture   Every metamonovalued reloid is monovalued.

Keywords:

Posted by porton
updated September 17th, 2013
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Topology

Every metamonovalued funcoid is monovalued ★★

Author(s): Porton

Conjecture   Every metamonovalued funcoid is monovalued.

The reverse is almost trivial: Every monovalued funcoid is metamonovalued.

Keywords: monovalued

Posted by porton
updated September 17th, 2013
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Topology

Decomposition of completions of reloids ★★

Author(s): Porton

Conjecture   For composable reloids $ f $ and $ g $ it holds
    \item $ \operatorname{Compl} ( g \circ f) = ( \operatorname{Compl} g) \circ f $ if $ f $ is a co-complete reloid; \item $ \operatorname{CoCompl} ( f \circ g) = f \circ \operatorname{CoCompl} g $ if $ f $ is a complete reloid; \item $ \operatorname{CoCompl} ( ( \operatorname{Compl} g) \circ f) = \operatorname{Compl} ( g \circ ( \operatorname{CoCompl} f)) = ( \operatorname{Compl} g) \circ ( \operatorname{CoCompl} f) $; \item $ \operatorname{Compl} ( g \circ ( \operatorname{Compl} f)) = \operatorname{Compl} ( g \circ f) $; \item $ \operatorname{CoCompl} ( ( \operatorname{CoCompl} g) \circ f) = \operatorname{CoCompl} ( g \circ f) $.

Keywords: co-completion; completion; reloid

Posted by porton
updated September 2nd, 2013
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