We define a new graph operator called the P3 intersection graph,
P3(G)- the intersection graph of all induced 3-paths in G. A characterization
of graphs G for which P-3 (G) is bipartite is given . Forbidden
subgraph characterization for P3 (G) having properties of being
chordal , H-free, complete are also obtained . For integers a and b
with a > 1 and b > a - 1, it is shown that there exists a graph G
such that X(G) = a, X(P3( G)) = b, where X is the chromatic number
of G. For the domination number -y(G), we construct graphs G such
that -y(G) = a and -y (P3(G)) = b for any two positive numbers a > 1
and b. Similar construction for the independence number and radius,
diameter relations are also discussed.
Balakrishna, N; Jacob, T M(Taylor & Francis, 2003)
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Abstract:
In this article it is proved that the stationary Markov sequences generated by minification models are ergodic and uniformly mixing. These results are used to establish the optimal properties of estimators for the parameters in the model. The problem of estimating the parameters in the exponential minification model is discussed in detail.
Bindhu M, Krishna; Manu P, John; Nandakumaran, V M(Indian Academy of Sciences, December , 2008)
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Abstract:
The chaotic dynamics of directly modulated semiconductor lasers with delayed optoelectronic feedback is studied numerically. The effects of positive and negative delayed optoelectronic feedback in producing chaotic outputs from such lasers with nonlinear gain reduction in its optimum value range is investigated using bifurcation diagrams. The results are confirmed by calculating the Lyapunov exponents. A negative delayed optoelectronic feedback configuration is found to be more effective in inducing chaotic dynamics to such systems with nonlinear gain reduction factor in the practical value range.
Usha, Nair; Bindu M, Krishna; Namboothiri, V N N; Nampoori, V P N(Springer, 2010)
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Abstract:
Machine tool chatter is an unfavorable phenomenon during metal cutting, which results in heavy vibration of cutting tool. With increase in depth of cut, the cutting regime changes from chatter-free cutting to one with chatter. In this paper, we propose the use of permutation entropy (PE), a conceptually simple and computationally fast measurement to detect the onset of chatter from the time series using sound signal recorded with a unidirectional microphone. PE can efficiently distinguish the regular and complex nature of any signal and extract information about the dynamics of the process by indicating sudden change in its value. Under situations where the data sets are huge and there is no time for preprocessing and fine-tuning, PE can effectively detect dynamical changes of the system. This makes PE an ideal choice for online detection of chatter, which is not possible with other conventional nonlinear methods. In the present study, the variation of PE under two cutting conditions is analyzed. Abrupt variation in the value of PE with increase in depth of cut indicates the onset of chatter vibrations. The results are verified using frequency spectra of the signals and the nonlinear measure, normalized coarse-grained information rate (NCIR).
Poly(ethylene terephthalate) (PET) based nanocomposites have been prepared with single walled carbon nanotubes
(SWNTs) through an ultrasound assisted dissolution-evaporation method. Differential scanning calorimetry studies
showed that SWNTs nucleate crystallization in PET at weight fractions as low as 0.3%, as the nanocomposite melt crystallized
during cooling at temperature 24 °C higher than neat PET of identical molecular weight. Isothermal crystallization
studies also revealed that SWNTs significantly accelerate the crystallization process. Mechanical properties of the PETSWNT
nanocomposites improved as compared to neat PET indicating the effective reinforcement provided by nanotubes
in the polymer matrix. Electrical conductivity measurements on the nanocomposite films showed that SWNTs at concentrations
exceeding 1 wt% in the PET matrix result in electrical percolation. Comparison of crystallization, conductivity and
transmission electron microscopy studies revealed that ultrasound assisted dissolution-evaporation method enables more
effective dispersion of SWNTs in the PET matrix as compared to the melt compounding method
Chitra, R N; Kuriakose, V C(American Institute of Physics, 2008)
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Abstract:
Synchronization in an array of mutually coupled systems with a finite time delay in coupling is studied using the Josephson junction as a model system. The sum of the transverse Lyapunov exponents is evaluated as a function of the parameters by linearizing the equation about the synchronization manifold. The dependence of synchronization on damping parameter, coupling constant, and time delay is studied numerically. The change in the dynamics of the system due to time delay and phase difference between the applied fields is studied. The case where a small frequency detuning between the applied fields is also discussed.
Mohanan,P; Girijavallabhan,C P; Aanandan,C K; K G Nair(Indian Journal of Radio & Space Physics, February , 1984)
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Abstract:
A forward - biased point contact germanium signal diode placed inside a waveguide section along the E -vector is found to
introduce significant phase shift of microwave signals . The usefulness of the arrangement as a phase modulator for microwave
carriers is demonstrated. While there is a less significant amplitude modulation accompanying phase modulation , the insertion
losses are found to be negligible. The observations can be explained on the basis of the capacitance variation of the barrier layer
with forward current in the diode
Chitra, R N; Kuriakose, V C(American Institute of Physics, 2008)
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Abstract:
We consider an array of N Josephson junctions connected in parallel and explore the condition for chaotic synchronization. It is found that the outer junctions can be synchronized while they remain uncorrelated to the inner ones when an external biasing is applied. The stability of the solution is found out for the outer junctions in the synchronization manifold. Symmetry considerations lead to a situation wherein the inner junctions can synchronize for certain values of the parameter. In the presence of a phase difference between the applied fields, all the junctions exhibit phase synchronization. It is also found that chaotic motion changes to periodic in the presence of phase differences.
Louis, Godfrey; Philip, J(The American Physical Society, 1996)
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Abstract:
A detailed ultrasonic study of the elastic properties of lithium ammonium sulfate ~LiNH4SO4! or LAS has
been carried out below room temperature. The elastic constants of LAS at room temperature are reported. The
discrepancy present in earlier elastic constant data associated with the different choice of axes for this orthorhombic
system are clarified. The results of the temperature variation study down to 220 K confirm the
ferroelastic phase transition at 285 K and establish a thermal hysteresis of about 2.5 K between the cooling and
heating cycles. Results of the investigation on the suspected weak phase transition at 256 K suggest that this
transition occurs at 242 K on cooling and at 256 K on heating, thus having a thermal hysteresis of about 14 K.
However, since the observed elastic anomaly for this transition is very small, the nature of this transition still
remains unclear
Ravi Kumar, A V; Padmaja, G; Nampoori, V P N; Vallabhan, C P G(Springer, November , 1989)
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Abstract:
Absorption spectra of formaldehyde molecule in the gas phase have been recorded using photoacoustic (PA) technique with pulsed dye laser at various power levels. The spectral profiles at higher power levels are found to be different from that obtained at lower laser powers. Two photon absorption (TPA) is found to be responsible for the photoacoustic signal at higher laser power while the absorption at lower laser power level is attributed to one photon absorption (OPA) process. Probable assignments for the different transitions are given in this paper.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Nibu, A George; George, A K(Journal of Physica: Condensed Matter, Institute of Physics Publishing, November 28, 2000)
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Abstract:
The thermal effusivity values in the isotropic phase of certain comb-shaped
polymers have been evaluated for the first time using an open photoacoustic
cell configuration. The compounds investigated have siloxane and acrylate
backbone and they carry mesogenic groups in their side chain. The results
indicate that the polymer chain length as well as the side chain length have
pronounced influence on the thermal effusivity values in liquid crystalline
polymers
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; Nibu, A George; George, A K(Journal of Physica: Condensed Matter, Institute of Physics Publishing, November 28, 2000)
[+]
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Abstract:
The thermal effusivity values in the isotropic phase of certain comb-shaped
polymers have been evaluated for the first time using an open photoacoustic
cell configuration. The compounds investigated have siloxane and acrylate
backbone and they carry mesogenic groups in their side chain. The results
indicate that the polymer chain length as well as the side chain length have
pronounced influence on the thermal effusivity values in liquid crystalline
polymers.
V P N Nampoori; Radhakrishnan, P; Girijavallabhan, C P; Nibu, A George; George, A K(Journal of Physica: Condensed Matter, Institute of Physics Publishing, November 28, 2000)
[+]
[-]
Abstract:
The thermal effusivity values in the isotropic phase of certain comb-shaped
polymers have been evaluated for the first time using an open photoacoustic
cell configuration. The compounds investigated have siloxane and acrylate
backbone and they carry mesogenic groups in their side chain. The results
indicate that the polymer chain length as well as the side chain length have
pronounced influence on the thermal effusivity values in liquid crystalline
polymers
Sajan D, George; Vallabhan, C P G; Heck, M; Radhakrishnan, P; Nampoori, V P N(Taylor & Francis, 2002)
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Abstract:
An open cell photoacoustic (PA) configuration has been employed to evaluate the thermal diffusivity of intrinsic InP as well as InP doped with tin and iron. Thermal diffusivity data have been evaluated from variation of phase of PA signal as a function of modulation frequency. In doped samples, we observe a reduced value for thermal diffusivity in comparison with intrinsic InP. We also observed that, while the phase of the PA signal varies linearly with the square root of chopping frequency for doped samples, the intrinsic material does not exhibit such behaviour in the experimental frequency range. These results have been interpreted in terms of the heat generation and phonon assisted heat diffusion mechanisms in semiconductors.
Sajan D, George; Aneeshkumar, B; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Society of Photo-Optical Instrumentation Engineers, 2004)
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Abstract:
An open-cell configuration of the photoacoustic (PA) technique is employed to determine the thermal and transport properties of intrinsic Si and Si doped with B (p-type) and P (n-type). The experimentally obtained phase of the PA signal under heat transmission configuration is fitted to that of theoretical model by taking thermal and transport properties, namely, thermal diffusivity, diffusion coefficient, and surface recombination velocity, as adjustable parameters. It is seen from the analysis that doping and also the nature of dopant have a strong influence on the thermal and transport properties of semiconductors. The results are interpreted in terms of the carrier-assisted and phonon-assisted heat transfer mechanisms in semiconductors as well as the various scattering processes occurring in the propagation of heat carriers.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; George, N A; Paul, T; Sebastian, M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
V P N Nampoori; Radhakrishnan, P; Girijavallabhan, C P; George, N A; Paul, T; Sebastian, M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; George, N A; Paul, T; Sebastian, M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Ritty J, Nedumpara; Binoy, Paul; Santhi, A; Radhakrishnan, P; Nampoori, V P N(Elsevier, January , 2004)
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Abstract:
In this paper, we present a laser-induced photoacoustic study on the photostability of laser dye Coumarin 540 doped in PMMA matrix and modified by the incorporation of low-molecular weight additives. The dependence of photostability of the dye on various experimental conditions, such as nature of solvents, incident optical power and dye concentration, is investigated in detail. The activation rates for the bleaching process are calculated for different concentrations and they suggest the possibility of two distinct mechanisms responsible for photodegradation. Further, analysis of the data confirms the linear dependence of photodegradation on the intensity of incident radiation. The role of different externally influencing parameters, such as wavelength and modulation frequency of incident radiation, is also discussed.