7,054 research outputs found
On The Panel Unit Root Tests Using Nonlinear Instrumental Variables
This paper re-examines the panel unit root tests proposed by Chang (2002). She establishes asymptotic independence of the t-statistics when integrable functions of lagged dependent variable are used as instruments even if the original series are cross sectionally dependent. She claims that her non-linear instrumental variable (NIV) panel unit root test is valid under general error cross correlations for any N (the cross section dimension) as T (the time dimension of the panel) tends to infinity. These results are largely due to her particular choice of the error correlation matrix which results in weak cross section dependence. Also, the asymptotic independence property of the t- statistics disappears when Chang's modified instruments are used. Using a common factor model with a sizeable degree of cross section correlations, we show that Chang's NIV panel unit root test suffers from gross size distortions, even when N is small relative to T
Fan-out in Gene Regulatory Networks
In synthetic biology, gene regulatory circuits are often constructed by
combining smaller circuit components. Connections between components are
achieved by transcription factors acting on promoters. If the individual
components behave as true modules and certain module interface conditions are
satisfied, the function of the composite circuits can in principle be
predicted. In this paper, we investigate one of the interface conditions:
fan-out. We quantify the fan-out, a concept widely used in electric
engineering, to indicate the maximum number of the downstream inputs that an
upstream output transcription factor can regulate. We show that the fan-out is
closely related to retroactivity studied by Del Vecchio, et al. We propose an
efficient operational method for measuring the fan-out that can be applied to
various types of module interfaces. We also show that the fan-out can be
enhanced by self-inhibitory regulation on the output. We discuss the potential
role of the inhibitory regulations found in gene regulatory networks and
protein signal pathways. The proposed estimation method for fanout not only
provides an experimentally efficient way for quantifying the level of
modularity in gene regulatory circuits but also helps characterize and design
module interfaces, enabling the modular construction of gene circuits.Comment: 28 pages, 5 figure
X-ray and EUV Observations of Simultaneous Short and Long Period Oscillations in Hot Coronal Arcade Loops
We report decaying quasi-periodic intensity oscillations in the X-ray (6-12
keV) and extreme ultraviolet (EUV) channels (131, 94, 1600, 304 \AA) observed
by the Fermi GBM (Gamma-ray Burst Monitor) and SDO/AIA, respectively, during a
C-class flare. The estimated period of oscillation and decay time in the X-ray
channel (6-12 keV) was about 202 s and 154 s, respectively. A similar
oscillation period was detected at the footpoint of the arcade loops in the AIA
1600 and 304 \AA channels. Simultaneously, AIA hot channels (94 and 131 \AA)
reveal propagating EUV disturbances bouncing back and forth between the
footpoints of the arcade loops. The period of the oscillation and decay time
were about 409 s and 1121 s, respectively. The characteristic phase speed of
the wave is about 560 km/s for about 115 Mm loop length, which is roughly
consistent with the sound speed at the temperature about 10-16 MK (480-608
km/s). These EUV oscillations are consistent with the SOHO/SUMER Doppler-shift
oscillations interpreted as the global standing slow magnetoacoustic wave
excited by a flare. The flare occurred at one of the footpoints of the arcade
loops, where the magnetic topology was a 3D fan-spine with a null-point.
Repetitive reconnection at this footpoint could cause the periodic acceleration
of non-thermal electrons that propagated to the opposite footpoint along the
arcade and precipitating there, causing the observed 202-s periodicity. Other
possible interpretations, e.g. the second harmonics of the slow mode are also
discussed.Comment: ApJ (in press), 13 pages, 6 figure
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