Enumerating tree-like polyphenyl isomers

NSFC [10831001]Enumeration of molecules is one of the fundamental problems in bioinformatics and plays an important role in drug discovery, experimental structure elucidation (e.g., by using NMR or mass spectrometry), molecular design and virtual library construction. We consider the enumeration of tree-like polyphenyls (C(6)nH(4n+2)). For this purpose, we de fine two generating functions T (x) and R (x) involving the numbers t(n) and r(n) of tree-like polyphenyls (TL-polyphenyls) and monosubstituted tree-like polyphenyls (MTL-polyphenyls), respectively. By characterizing the symmetry groups with respect to TL-polyphenyls and MTL-polyphenyls, we establish two functional equations for these two generating functions. This yields for the first time an efficient recursion formula for calculating the numbers t(n) and r(n). The two functional equations are also the fundamentals for analyzing their asymptotic behaviors, from which we derive the precise asymptotic values for both r(n) and t(n). The resulting asymptotic values are shown to fit well to the numerical results obtained by using our recursion formula. Finally, we give an explicit enumerating expression for TL-polyphenyls of a particular type: the linear polyphenyls

Characterization Results for the L(2, 1, 1)-Labeling Problem on Trees

An L(2, 1, 1)-labeling of a graph G is an assignment of non-negative integers (labels) to the vertices of G such that adjacent vertices receive labels with difference at least 2, and vertices at distance 2 or 3 receive distinct labels. The span of such a labelling is the difference between the maximum and minimum labels used, and the minimum span over all L(2, 1, 1)-labelings of G is called the L(2, 1, 1)-labeling number of G, denoted by λ2,1,1(G). It was shown by King, Ras and Zhou in [The L(h, 1, 1)-labelling problem for trees, European J. Combin. 31 (2010) 1295–1306] that every tree T has Δ2(T) − 1 ≤ λ2,1,1(T) ≤ Δ2(T), where Δ2(T) = maxuv∈E(T)(d(u) + d(v)). And they conjectured that almost all trees have the L(2, 1, 1)-labeling number attain the lower bound. This paper provides some sufficient conditions for λ2,1,1(T) = Δ2(T). Furthermore, we show that the sufficient conditions we provide are also necessary for trees with diameter at most 6

Characterization Results for the L(2, 1, 1)-Labeling Problem on Trees

An L(2, 1, 1)-labeling of a graph G is an assignment of non-negative integers (labels) to the vertices of G such that adjacent vertices receive labels with difference at least 2, and vertices at distance 2 or 3 receive distinct labels. The span of such a labelling is the difference between the maximum and minimum labels used, and the minimum span over all L(2, 1, 1)-labelings of G is called the L(2, 1, 1)-labeling number of G, denoted by λ2,1,1(G). It was shown by King, Ras and Zhou in [The L(h, 1, 1)-labelling problem for trees, European J. Combin. 31 (2010) 1295–1306] that every tree T has Δ2(T) − 1 ≤ λ2,1,1(T) ≤ Δ2(T), where Δ2(T) = maxuv∈E(T)(d(u) + d(v)). And they conjectured that almost all trees have the L(2, 1, 1)-labeling number attain the lower bound. This paper provides some sufficient conditions for λ2,1,1(T) = Δ2(T). Furthermore, we show that the sufficient conditions we provide are also necessary for trees with diameter at most 6

Antimagic Labeling of Some Biregular Bipartite Graphs

An antimagic labeling of a graph G = (V, E) is a one-to-one mapping from E to {1, 2, . . ., |E|} such that distinct vertices receive different label sums from the edges incident to them. G is called antimagic if it admits an antimagic labeling. It was conjectured that every connected graph other than K2 is antimagic. The conjecture remains open though it was verified for several classes of graphs such as regular graphs. A bipartite graph is called (k, k′)-biregular, if each vertex of one of its parts has the degree k, while each vertex of the other parts has the degree k′. This paper shows the following results. (1) Each connected (2, k)-biregular (k ≥ 3) bipartite graph is antimagic; (2) Each (k, pk)-biregular (k ≥ 3, p ≥ 2) bipartite graph is antimagic; (3) Each (k, k2 + y)-biregular (k ≥ 3, y ≥ 1) bipartite graph is antimagic

Recent progress on strong edge-coloring of graphs

A strong edge-coloring of a graph G = (V,E) is a partition of its edge set E into induced matchings. In this paper, we gave a short survey on recent results about strong edge-coloring of a graph

On Critical Circuits in k-Connected Matroids

We show that, for every integer k≥ 4 , if M is a k-connected matroid and C is a circuit of M such that for every e∈ C, M\ e is not k-connected, then C meets a cocircuit of size at most 2 k- 3 ; furthermore, if M is binary and k≥ 5 , then C meets a cocircuit of size at most 2 k- 4. It follows from our results and a result of Reid et al [5] that every minimally k-connected matroid has a cocircuit of size at most 2 k- 3 , and every minimally k-connected binary matroid has a cocircuit of size at most 2 k- 4

On Critical Circuits in k-Connected Matroids

We show that, for every integer k≥ 4 , if M is a k-connected matroid and C is a circuit of M such that for every e∈ C, M\ e is not k-connected, then C meets a cocircuit of size at most 2 k- 3 ; furthermore, if M is binary and k≥ 5 , then C meets a cocircuit of size at most 2 k- 4. It follows from our results and a result of Reid et al [5] that every minimally k-connected matroid has a cocircuit of size at most 2 k- 3 , and every minimally k-connected binary matroid has a cocircuit of size at most 2 k- 4

Strong edge-coloring for planar graphs with large girth

A strong edge-coloring of a graph [Formula presented] is a partition of its edge set [Formula presented] into induced matchings. Let [Formula presented] be a connected planar graph with girth [Formula presented] and maximum degree [Formula presented]. We show that either [Formula presented] is isomorphic to a subgraph of a very special [Formula presented]-regular graph with girth [Formula presented], or [Formula presented] has a strong edge-coloring using at most [Formula presented] colors

Enumerating stereo-isomers of tree-like polyinositols

NSFC [11271307]Enumeration of molecules is one of the fundamental problems in bioinformatics and chemoinformatics which is also important from a practical viewpoint. We consider the problem of enumerating the stereo-isomers of tree-like polyinositol molecules (with chemical formula where is the number of hexagonal oinositol rings) and monosubstituted tree-like polyinositols (with chemical formula ). We establish recursion counting formulas for the numbers of the stereo-isomers for these two classes of molecules, in which chirality is also taken into account. In our study, the generating function, Plya enumeration theory and 'Dissimilarity Characteristic Theorem' play important roles. Compared to some known computer programs such as ISOMERS, MOLGEN, exhaustive construction and Dynamic Programming etc., our method is more efficient to our enumeration problem with larger number of inositol rings. Further more, based on the obtained recursion formulas, we derive the asymptotic values for the numbers of these two stereo-isomers from which we conclude that almost all tree-like and monosubstituted tree-like polyinositols are chiral

DETERMINING THE COMPONENT NUMBER OF LINKS CORRESPONDING TO TRIANGULAR AND HONEYCOMB LATTICES

NSFC [10831001]; Fundamental Research Funds for the Central Universities [2010121007]There is a classical correspondence between edge-signed plane graphs and link diagrams. Determining component number of links corresponding to plane graphs may be one of the first problems in studying links by using graphs. There has been several early studies in this aspect, for example, the component number of links formed from 2-dimensional square lattices (4(4)) has been determined. In this paper, we determine the component number of links corresponding to 2-dimensional triangular (3(6)) and honeycomb (6(3)) lattices with free or cyclic boundary condition