3,064 research outputs found
Extremal numbers for odd cycles
We describe the C_{2k+1}-free graphs on n vertices with maximum number of
edges. The extremal graphs are unique except for n = 3k-1, 3k, 4k-2, or 4k-1.
The value of ex(n,C_{2k+1}) can be read out from the works of Bondy, Woodall,
and Bollobas, but here we give a new streamlined proof. The complete
determination of the extremal graphs is also new.
We obtain that the bound for n_0(C_{2k+1}) is 4k in the classical theorem of
Simonovits, from which the unique extremal graph is the bipartite Turan graph.Comment: 6 page
Model for Triplet State Engineering in Organic Light Emitting Diodes
Engineering the position of the lowest triplet state (T1) relative to the
first excited singlet state (S1) is of great importance in improving the
efficiencies of organic light emitting diodes and organic photovoltaic cells.
We have carried out model exact calculations of substituted polyene chains to
understand the factors that affect the energy gap between S1 and T1. The
factors studied are backbone dimerisation, different donor-acceptor
substitutions and twisted geometry. The largest system studied is an eighteen
carbon polyene which spans a Hilbert space of about 991 million. We show that
for reverse intersystem crossing (RISC) process, the best system involves
substituting all carbon sites on one half of the polyene with donors and the
other half with acceptors.Comment: 7 Pages; 10 Figure
Universality and m_X cut effects in B -> Xs l+ l-
The most precise comparison between theory and experiment for the B -> Xs l+
l- rate is in the low q^2 region, but the hadronic uncertainties associated
with an experimentally required cut on m_X potentially spoil the search for new
physics in these decays. We show that a 10-30% reduction of d\Gamma(B -> Xs l+
l-) / dq^2 due to the m_X cut can be accurately computed using the B -> X_s
gamma shape function. The effect is universal for all short distance
contributions in the limit m_X^2 << m_B^2, and this universality is spoiled
neither by realistic values of the m_X cut nor by alpha_s corrections. Both the
differential decay rate and forward-backward asymmetry with an m_X cut are
computed.Comment: 5 pages, journal versio
Extracting short distance information from b-->s[script-l]+[script-l]- effectively
We point out that in inclusive B-->Xs[script-l]+[script-l]- decay an angular decomposition provides a third (q2 dependent) observable sensitive to a different combination of Wilson coefficients than the rate and the forward-backward asymmetry. Since a precise measurement of q2 dependence requires large data sets, it is important to consider the data integrated over regions of q2. We develop a strategy to extract all measurable Wilson coefficients in B-->Xs[script-l]+[script-l]- from a few simple integrated rates in the low q2 region. A similar decomposition in B-->K*[script-l]+[script-l]-, together with the B-->K*gamma rate, also provides a determination of the Wilson coefficients, without reliance on form factor models and without having to measure the zero of the forward-backward asymmetry
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