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Helioseismology: a fantastic tool to probe the interior of the Sun
Helioseismology, the study of global solar oscillations, has proved to be an
extremely powerful tool for the investigation of the internal structure and
dynamics of the Sun. Studies of time changes in frequency observations of solar
oscillations from helioseismology experiments on Earth and in space have shown,
for example, that the Sun's shape varies over solar cycle timescales. In
particular, far-reaching inferences about the Sun have been obtained by
applying inversion techniques to observations of frequencies of oscillations.
The results, so far, have shown that the solar structure is remarkably close to
the predictions of the standard solar model and, recently, that the
near-surface region can be probed with sufficiently high spatial resolution as
to allow investigations of the equation of state and of the solar envelope
helium abundance. The same helioseismic inversion methods can be applied to the
rotational frequency splittings to deduce with high accuracy the internal
rotation velocity of the Sun, as function of radius and latitude. This also
allows us to study some global astrophysical properties of the Sun, such as the
angular momentum, the grativational quadrupole moment and the effect of
distortion induced on the surface (oblateness). The helioseismic approach and
what we have learnt from it during the last decades about the interior of the
Sun are reviewed here.Comment: 36 page
Abstract densities and ideals of sets
Abstract upper densities are monotone and subadditive functions from the
power set of positive integers to the unit real interval that generalize the
upper densities used in number theory, including the upper asymptotic density,
the upper Banach density, and the upper logarithmic density. We answer a
question posed by G. Grekos in 2013, and prove the existence of translation
invariant abstract upper densities onto the unit interval, whose null sets are
precisely the family of finite sets, or the family of sequences whose series of
reciprocals converge. We also show that no such density can be atomless. (More
generally, these results also hold for a large class of summable ideals.
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