627,061 research outputs found
Four Degrees of Separation, Really
We recently measured the average distance of users in the Facebook graph,
spurring comments in the scientific community as well as in the general press
("Four Degrees of Separation"). A number of interesting criticisms have been
made about the meaningfulness, methods and consequences of the experiment we
performed. In this paper we want to discuss some methodological aspects that we
deem important to underline in the form of answers to the questions we have
read in newspapers, magazines, blogs, or heard from colleagues. We indulge in
some reflections on the actual meaning of "average distance" and make a number
of side observations showing that, yes, 3.74 "degrees of separation" are really
few
Four Degrees of Separation
Frigyes Karinthy, in his 1929 short story "L\'aancszemek" ("Chains")
suggested that any two persons are distanced by at most six friendship links.
(The exact wording of the story is slightly ambiguous: "He bet us that, using
no more than five individuals, one of whom is a personal acquaintance, he could
contact the selected individual [...]". It is not completely clear whether the
selected individual is part of the five, so this could actually allude to
distance five or six in the language of graph theory, but the "six degrees of
separation" phrase stuck after John Guare's 1990 eponymous play. Following
Milgram's definition and Guare's interpretation, we will assume that "degrees
of separation" is the same as "distance minus one", where "distance" is the
usual path length-the number of arcs in the path.) Stanley Milgram in his
famous experiment challenged people to route postcards to a fixed recipient by
passing them only through direct acquaintances. The average number of
intermediaries on the path of the postcards lay between 4.4 and 5.7, depending
on the sample of people chosen.
We report the results of the first world-scale social-network graph-distance
computations, using the entire Facebook network of active users (\approx721
million users, \approx69 billion friendship links). The average distance we
observe is 4.74, corresponding to 3.74 intermediaries or "degrees of
separation", showing that the world is even smaller than we expected, and
prompting the title of this paper. More generally, we study the distance
distribution of Facebook and of some interesting geographic subgraphs, looking
also at their evolution over time.
The networks we are able to explore are almost two orders of magnitude larger
than those analysed in the previous literature. We report detailed statistical
metadata showing that our measurements (which rely on probabilistic algorithms)
are very accurate
Study of lee-side flows over conically cambered delta wings at supersonic speeds, part 1
An experimental investigation was performed in which surface pressure data, flow visualization data, and force and moment data were obtained on four conical delta wing models which differed in leading-edge camber only. Wing leading-edge camber was achieved through a deflection of the outboard 30% of the local wind semispan of a reference 75 degrees swept flat delta wing. The four wing models have leading-edge deflection angles delta sub F of 0, 5, 10, and 15 degrees measured streamwise. Data for the wings with delta sub F = 10 and 15 degrees showed that hinge-line separation dominated the lee-side wing loading and prohibited the develpment of leading-edge separation on the deflected portion of wing leading edge. However, data for the wing with delta sub F = 5 degrees, a vortex was positioned on the deflected leading edge with reattachment at the hinge line. Flow visualization results were presented which detail the influence of Mach number, angle of attack, and camber on the lee-side flow characteristics of conically cambered delta wings. Analysis of photgraphic data identified the existence of 12 distinctive lee-side flow types. In general, the aerodynamic force and moment data correlated well with the pressure and flow visualization data
Evaluation of panel code predictions with experimental results of inlet performance for a 17-inch ducted prop/fab simulator operating at Mach 0.2
An axisymmetric panel code was used to evaluate a series of ducted propeller inlets. The inlets were tested in the Lewis 9 by 15 Foot Low Speed Wind Tunnel. Three basic inlets having ratios of shroud length to propeller diameter of 0.2, 0.4, and 0.5 were tested with the Pratt and Whitney ducted prop/fan simulator. A fourth hybrid inlet consisting of the shroud from the shortest basic inlet coupled with the spinner from the largest basic inlet was also tested. This later configuration represented the shortest overall inlet. The simulator duct diameter at the propeller face was 17.25 inches. The short and long spinners provided hub-to-tip ratios of 0.44 at the propeller face. The four inlets were tested at a nominal free stream Mach number of 0.2 and at angles of attack from 0 degrees to 35 degrees. The panel code method incorporated a simple two-part separation model which yielded conservative estimates of inlet separation
What are the Best Hierarchical Descriptors for Complex Networks?
This work reviews several hierarchical measurements of the topology of
complex networks and then applies feature selection concepts and methods in
order to quantify the relative importance of each measurement with respect to
the discrimination between four representative theoretical network models,
namely Erd\"{o}s-R\'enyi, Barab\'asi-Albert, Watts-Strogatz as well as a
geographical type of network. The obtained results confirmed that the four
models can be well-separated by using a combination of measurements. In
addition, the relative contribution of each considered feature for the overall
discrimination of the models was quantified in terms of the respective weights
in the canonical projection into two dimensions, with the traditional
clustering coefficient, hierarchical clustering coefficient and neighborhood
clustering coefficient resulting particularly effective. Interestingly, the
average shortest path length and hierarchical node degrees contributed little
for the separation of the four network models.Comment: 9 pages, 4 figure
Tachoastrometry: astrometry with radial velocities
Spectra of composite systems (e.g., spectroscopic binaries) contain spatial
information that can be retrieved by measuring the radial velocities (i.e.,
Doppler shifts) of the components in four observations with the slit rotated by
90 degrees in the sky. By using basic concepts of slit spectroscopy we show
that the geometry of composite systems can be reliably retrieved by measuring
only radial velocity differences taken with different slit angles. The spatial
resolution is determined by the precision with which differential radial
velocities can be measured. We use the UVES spectrograph at the VLT to observe
the known spectroscopic binary star HD 188088 (HIP 97944), which has a maximum
expected separation of 23 milli-arcseconds. We measure an astrometric signal in
radial velocity of 276 \ms, which corresponds to a separation between the two
components at the time of the observations of 18 milli-arcseconds. The
stars were aligned east-west. We describe a simple optical device to
simultaneously record pairs of spectra rotated by 180 degrees, thus reducing
systematic effects. We compute and provide the function expressing the shift of
the centroid of a seeing-limited image in the presence of a narrow slit.The
proposed technique is simple to use and our test shows that it is amenable for
deriving astrometry with milli-arcsecond accuracy or better, beyond the
diffraction limit of the telescope. The technique can be further improved by
using simple devices to simultaneously record the spectra with 180 degrees
angles.With tachoastrometry, radial velocities and astrometric positions can be
measured simultaneously for many double line system binaries in an easy way.
The method is not limited to binary stars, but can be applied to any
astrophysical configuration in which spectral lines are generated by separate
(non-rotational symmetric) regions.Comment: Accepted for publication in A&
The Effect of Depression on Pattern Separation Task Performance
The aim of this study was to investigate a possible relationship between depression and memory performance, hypothesizing a decrease in performance among participants with depression tendencies. The cognitive memory task of pattern separation was tested with four different levels of difficulty by use of a computerized memory test and the self-rating MADRS-S depression scale was used to assess the degrees of depression among the non-clinical sample of participants. Pattern separation, the cognitive ability of processing new input and distinguishing it from already stored information was tested on four levels on all participants. Participants were 40 students from Lund University, 26 women and 14 men, with a mean age of 24 (SD 4.19). The levels of depression were then analyzed and compared to the pattern separation task performance. The results proved the experiments ability to successfully test four levels of pattern separation demand, as every level showed an increase in difficulty. Although no significance was found between pattern separation and degree of depression, the results showed a tendency toward the hypothesized direction
- …