10 research outputs found
Tight upper bound on the maximum anti-forcing numbers of graphs
Let be a simple graph with a perfect matching. Deng and Zhang showed that
the maximum anti-forcing number of is no more than the cyclomatic number.
In this paper, we get a novel upper bound on the maximum anti-forcing number of
and investigate the extremal graphs. If has a perfect matching
whose anti-forcing number attains this upper bound, then we say is an
extremal graph and is a nice perfect matching. We obtain an equivalent
condition for the nice perfect matchings of and establish a one-to-one
correspondence between the nice perfect matchings and the edge-involutions of
, which are the automorphisms of order two such that and
are adjacent for every vertex . We demonstrate that all extremal
graphs can be constructed from by implementing two expansion operations,
and is extremal if and only if one factor in a Cartesian decomposition of
is extremal. As examples, we have that all perfect matchings of the
complete graph and the complete bipartite graph are nice.
Also we show that the hypercube , the folded hypercube ()
and the enhanced hypercube () have exactly ,
and nice perfect matchings respectively.Comment: 15 pages, 7 figure
Relations between global forcing number and maximum anti-forcing number of a graph
The global forcing number of a graph G is the minimal cardinality of an edge
subset discriminating all perfect matchings of G, denoted by gf(G). For any
perfect matching M of G, the minimal cardinality of an edge subset S in E(G)-M
such that G-S has a unique perfect matching is called the anti-forcing number
of M,denoted by af(G, M). The maximum anti-forcing number of G among all
perfect matchings is denoted by Af(G). It is known that the maximum
anti-forcing number of a hexagonal system equals the famous Fries number.
We are interested in some comparisons between the global forcing number and
the maximum anti-forcing number of a graph. For a bipartite graph G, we show
that gf(G)is larger than or equal to Af(G). Next we mainly extend such result
to non-bipartite graphs, which is the set of all graphs with a perfect matching
which contain no two disjoint odd cycles such that their deletion results in a
subgraph with a perfect matching. For any such graph G, we also have gf(G) is
larger than or equal to Af(G) by revealing further property of non-bipartite
graphs with a unique perfect matching. As a consequence, this relation also
holds for the graphs whose perfect matching polytopes consist of non-negative
1-regular vectors. In particular, for a brick G, de Carvalho, Lucchesi and
Murty [4] showed that G satisfying the above condition if and only if G is
solid, and if and only if its perfect matching polytope consists of
non-negative 1-regular vectors.
Finally, we obtain tight upper and lower bounds on gf(G)-Af(G). For a
connected bipartite graph G with 2n vertices, we have that 0 \leq gf(G)-Af(G)
\leq 1/2 (n-1)(n-2); For non-bipartite case, -1/2 (n^2-n-2) \leq gf(G)-Af(G)
\leq (n-1)(n-2).Comment: 19 pages, 11 figure
Fractional forcing number of graphs
The notion of forcing sets for perfect matchings was introduced by Harary,
Klein, and \v{Z}ivkovi\'{c}. The application of this problem in chemistry, as
well as its interesting theoretical aspects, made this subject very active. In
this work, we introduce the notion of the forcing function of fractional
perfect matchings which is continuous analogous to forcing sets defined over
the perfect matching polytope of graphs. We show that our defined object is a
continuous and concave function extension of the integral forcing set. Then, we
use our results about this extension to conclude new bounds and results about
the integral case of forcing sets for the family of edge and vertex-transitive
graphs and in particular, hypercube graphs