8,580 research outputs found
2-Domination and Annihilation Numbers
Using information provided by Ryan Pepper and Ermelinda DeLaVina in their paper On the 2-Domination number and Annihilation Number, I developed a new bound on the 2- domination number of trees. An original bound, γ2(G) ≤ (n+n1)/ 2 , had been shown by many other authors. Our goal was to generate a tighter bound in some cases and work towards generating a more general bound on the 2-domination number for all graphs. Throughout the span of this project I generated and proved the bound γ2(T ) ≤ 1/3 (n + 2n1 + n2). To prove this bound I first proved that the 2-domination number of a tree was less than or equal to the sum of two sub-trees formed by the deletion of an edge: γ2(T ) ≤ γ2(T1) + γ2(T2). From there, I proved our bound by showing that a minimum counter-example did not exist. A large portion of the results involves cases where a graph T is considered the minimum counter-example for the sake of contradiction. From there, I showed that if T was a counter-example, then a sub-tree T1 was also a counter-example, meaning that T would no longer be the minimum counter-example. The last portion of the results is a section comparing the two bounds on the 2-domination number with respect to the number of edges and the degrees of those edges
A characterization of trees with equal 2-domination and 2-independence numbers
A set of vertices in a graph is a -dominating set if every vertex
of not in is adjacent to at least two vertices in , and is a
-independent set if every vertex in is adjacent to at most one vertex of
. The -domination number is the minimum cardinality of a
-dominating set in , and the -independence number is the
maximum cardinality of a -independent set in . Chellali and Meddah [{\it
Trees with equal -domination and -independence numbers,} Discussiones
Mathematicae Graph Theory 32 (2012), 263--270] provided a constructive
characterization of trees with equal -domination and -independence
numbers. Their characterization is in terms of global properties of a tree, and
involves properties of minimum -dominating and maximum -independent sets
in the tree at each stage of the construction. We provide a constructive
characterization that relies only on local properties of the tree at each stage
of the construction.Comment: 17 pages, 4 figure
Improved constraints on the primordial power spectrum at small scales from ultracompact minihalos
For a Gaussian spectrum of primordial density fluctuations, ultracompact
minihalos (UCMHs) of dark matter are expected to be produced in much greater
abundance than, e.g., primordial black holes. Forming shortly after
matter-radiation equality, these objects would develop very dense and spiky
dark matter profiles. In the standard scenario where dark matter consists of
thermally-produced, weakly-interacting massive particles, UCMHs could thus
appear as highly luminous gamma-ray sources, or leave an imprint in the cosmic
microwave background by changing the reionisation history of the Universe. We
derive corresponding limits on the cosmic abundance of UCMHs at different
epochs, and translate them into constraints on the primordial power spectrum.
We find the resulting constraints to be quite severe, especially at length
scales much smaller than what can be directly probed by the cosmic microwave
background or large-scale structure observations. We use our results to provide
an updated compilation of the best available constraints on the power of
density fluctuations on all scales, ranging from the present-day horizon to
scales more than 20 orders of magnitude smaller.Comment: 7 figures, 14 pages + appendices. v2 matches version accepted for
publication in PRD; updated to WMAP normalisation, updated reionisation
limits, various other small changes. v3 slightly corrects the normalisation
used for displaying past data in Fig 6, as well as a sign typo picked up in
proof in Eq 26. All results and conclusions completely unchange
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