362 research outputs found
Graph Coloring Problems and Group Connectivity
1. Group connectivity. Let A be an abelian group and let iA(G) be the smallest positive integer m such that Lm(G) is A-connected. A path P of G is a normal divalent path if all internal vertices of P are of degree 2 in G and if |E(P)|= 2, then P is not in a 3-cycle of G. Let l(G) = max{lcub}m : G has a normal divalent path of length m{rcub}. We obtain the following result. (i) If |A| ≥ 4, then iA( G) ≤ l(G). (ii) If | A| ≥ 4, then iA(G) ≤ |V(G)| -- Delta(G). (iii) Suppose that |A| ≥ 4 and d = diam( G). If d ≤ |A| -- 1, then iA(G) ≤ d; and if d ≥ |A|, then iA(G) ≤ 2d -- |A| + 1. (iv) iZ 3 (G) ≤ l(G) + 2. All those bounds are best possible.;2. Modulo orientation. A mod (2p + 1)-orientation D is an orientation of G such that d +D(v) = d--D(v) (mod 2p + 1) for any vertex v ∈ V ( G). We prove that for any integer t ≥ 2, there exists a finite family F = F(p, t) of graphs that do not have a mod (2p + 1)-orientation, such that every graph G with independence number at most t either admits a mod (2p+1)-orientation or is contractible to a member in F. In particular, the graph family F(p, 2) is determined, and our results imply that every 8-edge-connected graph G with independence number at most two admits a mod 5-orientation.;3. Neighbor sum distinguishing total coloring. A proper total k-coloring &phis; of a graph G is a mapping from V(G) ∪ E(G) to {lcub}1,2, . . .,k{rcub} such that no adjacent or incident elements in V(G) ∪ E( G) receive the same color. Let m&phis;( v) denote the sum of the colors on the edges incident with the vertex v and the color on v. A proper total k-coloring of G is called neighbor sum distinguishing if m &phis;(u) ≠m&phis;( v) for each edge uv ∈ E( G ). Let chitSigma(G) be the neighbor sum distinguishing total chromatic number of a graph G. Pilsniak and Wozniak conjectured that for any graph G, chitSigma( G) ≤ Delta(G) + 3. We show that if G is a graph with treewidth ℓ ≥ 3 and Delta(G) ≥ 2ℓ + 3, then chitSigma( G) + ℓ -- 1. This upper bound confirms the conjecture for graphs with treewidth 3 and 4. Furthermore, when ℓ = 3 and Delta ≥ 9, we show that Delta(G)+1 ≤ chit Sigma(G) ≤ Delta(G)+2 and characterize graphs with equalities.;4. Star edge coloring. A star edge coloring of a graph is a proper edge coloring such that every connected 2-colored subgraph is a path with at most 3 edges. Let ch\u27st(G) be the list star chromatic index of G: the minimum s such that for every s-list assignment L for the edges, G has a star edge coloring from L. By introducing a stronger coloring, we show with a very concise proof that the upper bound of the star chromatic index of trees also holds for list star chromatic index of trees, i.e. ch\u27st( T) ≤ [3Delta/2] for any tree T with maximum degree Delta. And then by applying some orientation technique we present two upper bounds for list star chromatic index of k-degenerate graphs
Progress on the adjacent vertex distinguishing edge colouring conjecture
A proper edge colouring of a graph is adjacent vertex distinguishing if no
two adjacent vertices see the same set of colours. Using a clever application
of the Local Lemma, Hatami (2005) proved that every graph with maximum degree
and no isolated edge has an adjacent vertex distinguishing edge
colouring with colours, provided is large enough. We
show that this bound can be reduced to . This is motivated by the
conjecture of Zhang, Liu, and Wang (2002) that colours are enough
for .Comment: v2: Revised following referees' comment
Graph Coloring via Degeneracy in Streaming and Other Space-Conscious Models
We study the problem of coloring a given graph using a small number of colors
in several well-established models of computation for big data. These include
the data streaming model, the general graph query model, the massively parallel
computation (MPC) model, and the CONGESTED-CLIQUE and the LOCAL models of
distributed computation. On the one hand, we give algorithms with sublinear
complexity, for the appropriate notion of complexity in each of these models.
Our algorithms color a graph using about colors, where
is the degeneracy of : this parameter is closely related to the
arboricity . As a function of alone, our results are
close to best possible, since the optimal number of colors is .
On the other hand, we establish certain lower bounds indicating that
sublinear algorithms probably cannot go much further. In particular, we prove
that any randomized coloring algorithm that uses many colors,
would require storage in the one pass streaming model, and
many queries in the general graph query model, where is the
number of vertices in the graph. These lower bounds hold even when the value of
is known in advance; at the same time, our upper bounds do not
require to be given in advance.Comment: 26 page
Phase transition in the sample complexity of likelihood-based phylogeny inference
Reconstructing evolutionary trees from molecular sequence data is a
fundamental problem in computational biology. Stochastic models of sequence
evolution are closely related to spin systems that have been extensively
studied in statistical physics and that connection has led to important
insights on the theoretical properties of phylogenetic reconstruction
algorithms as well as the development of new inference methods. Here, we study
maximum likelihood, a classical statistical technique which is perhaps the most
widely used in phylogenetic practice because of its superior empirical
accuracy.
At the theoretical level, except for its consistency, that is, the guarantee
of eventual correct reconstruction as the size of the input data grows, much
remains to be understood about the statistical properties of maximum likelihood
in this context. In particular, the best bounds on the sample complexity or
sequence-length requirement of maximum likelihood, that is, the amount of data
required for correct reconstruction, are exponential in the number, , of
tips---far from known lower bounds based on information-theoretic arguments.
Here we close the gap by proving a new upper bound on the sequence-length
requirement of maximum likelihood that matches up to constants the known lower
bound for some standard models of evolution.
More specifically, for the -state symmetric model of sequence evolution on
a binary phylogeny with bounded edge lengths, we show that the sequence-length
requirement behaves logarithmically in when the expected amount of mutation
per edge is below what is known as the Kesten-Stigum threshold. In general, the
sequence-length requirement is polynomial in . Our results imply moreover
that the maximum likelihood estimator can be computed efficiently on randomly
generated data provided sequences are as above.Comment: To appear in Probability Theory and Related Field
- …