578 research outputs found
Multicast Network Coding and Field Sizes
In an acyclic multicast network, it is well known that a linear network
coding solution over GF() exists when is sufficiently large. In
particular, for each prime power no smaller than the number of receivers, a
linear solution over GF() can be efficiently constructed. In this work, we
reveal that a linear solution over a given finite field does \emph{not}
necessarily imply the existence of a linear solution over all larger finite
fields. Specifically, we prove by construction that: (i) For every source
dimension no smaller than 3, there is a multicast network linearly solvable
over GF(7) but not over GF(8), and another multicast network linearly solvable
over GF(16) but not over GF(17); (ii) There is a multicast network linearly
solvable over GF(5) but not over such GF() that is a Mersenne prime
plus 1, which can be extremely large; (iii) A multicast network linearly
solvable over GF() and over GF() is \emph{not} necessarily
linearly solvable over GF(); (iv) There exists a class of
multicast networks with a set of receivers such that the minimum field size
for a linear solution over GF() is lower bounded by
, but not every larger field than GF() suffices to
yield a linear solution. The insight brought from this work is that not only
the field size, but also the order of subgroups in the multiplicative group of
a finite field affects the linear solvability of a multicast network
Turbulent entrainment origin of protostellar outflows
Protostellar outflow is a prominent process that accompanies the formation of
stars. It is generally agreed that wide-angled protostellar outflows come from
the interaction between the wind from a forming star and the ambient gas.
However, it is still unclear how the interaction takes place. In this work, we
theoretically investigate the possibility that the outflow results from
interaction between the wind and the ambient gas in the form of turbulent
entrainment. In contrast to the previous models, turbulent motion of the
ambient gas around the protostar is taken into account. In our model, the
ram-pressure of the wind balances the turbulent ram-pressure of the ambient
gas, and the outflow consists of the ambient gas entrained by the wind. The
calculated outflow from our modelling exhibits a conical shape. The total mass
of the outflow is determined by the turbulent velocity of the envelope as well
as the outflow age, and the velocity of the outflow is several times higher
than the velocity dispersion of the ambient gas. The outflow opening angle
increases with the strength of the wind and decreases with the increasing
ambient gas turbulence. The outflow exhibits a broad line width at every
position. We propose that the turbulent entrainment process, which happens
ubiquitously in nature, plays a universal role in shaping protostellar
outflows.Comment: 15 pages, accepted for publication in A&
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