The Lifecycles of Apps in a Social Ecosystem

Apps are emerging as an important form of on-line content, and they combine aspects of Web usage in interesting ways --- they exhibit a rich temporal structure of user adoption and long-term engagement, and they exist in a broader social ecosystem that helps drive these patterns of adoption and engagement. It has been difficult, however, to study apps in their natural setting since this requires a simultaneous analysis of a large set of popular apps and the underlying social network they inhabit. In this work we address this challenge through an analysis of the collection of apps on Facebook Login, developing a novel framework for analyzing both temporal and social properties. At the temporal level, we develop a retention model that represents a users tendency to return to an app using a very small parameter set. At the social level, we organize the space of apps along two fundamental axes --- popularity and sociality --- and we show how a users probability of adopting an app depends both on properties of the local network structure and on the match between the users attributes, his or her friends attributes, and the dominant attributes within the apps user population. We also develop models that show the importance of different feature sets with strong performance in predicting app success.

On the long and short nulls, modes and interpulse emission of radio pulsar B1944+17

We present a single pulse study of pulsar B1944+17, whose non-random nulls dominate nearly 70% of its pulses and usually occur at mode boundaries. When not in the null state, this pulsar displays four bright modes of emission, three of which exhibit drifting subpulses. B1944+17 displays a weak interpulse whose position relative to the main pulse we find to be frequency independent. Its emission is nearly 100% polarized, its polarization-angle traverse is very shallow and opposite in direction to that of the main pulse, and it nulls approximately two-thirds of the time. Geometric modeling indicates that this pulsar is a nearly aligned rotator whose alpha value is hardly 2 degrees--i.e., its magnetic axis is so closely aligned with its rotation axis that its sightline orbit remains within its conal beam. The stars nulls appear to be of two distinct types: those with lengths less than about 8 rotation periods appear to be pseudonulls--that is, produced by empty sightline traverses through the conal beam system; whereas the longer nulls appear to represent actual cessations of the pulsars emission engine.