Diversity and mobility in education and work present a paramount challenge that needs better conceptualization in educational theory. This challenge has been addressed by educational scholars with the notion of boundaries, particularly by the concepts of boundary crossing and boundary objects. Although studies on boundary crossing and boundary objects emphasize that boundaries carry learning potential, it is not explicated in what way they do so. By reviewing this literature, this article offers an understanding of boundaries as dialogical phenomena. The review of the literature reveals four potential learning mechanisms that can take place at boundaries: identification, coordination, reflection, and transformation. These mechanisms show various ways in which sociocultural differences and resulting discontinuities in action and interaction can come to function as resources for development of intersecting identities and practices.

Most current P2P file sharing systems treat their users as anonymous, unrelated entities, and completely disregard any social relationships between them. However, social phenomena such as friendship and the existence of communities of users with similar tastes may be well exploited in such systems, to increase their usability and performance. In this paper we present a novel social-based P2P file-sharing paradigm that exploits social phenomena by maintaining social networks and using these in content discovery, content recommendation, and downloading. Based on this paradigm's first class concepts such as taste groups, friends, and friends-offriends, we have designed and implemented the TRIBLER P2P filesharing system as a set of extensions to Bittorrent. We present and discuss the design of TRIBLER, and we show evidence that TRIBLER enables fast, trusted content discovery and recommendation at a low additional overhead, and a significant improvement in download performance.

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A pandemic is currently grinding public life to a halt. Schools and universities are closed in many countries. Conferences are canceled or postponed. We empathize with people who have lost dear ones or turned ill, and with those who struggle to keep life going, for themselves and their dear ones. Organizers of many conferences have seen their huge efforts evaporate by one single decision, necessary to slow down the spread of the Covid-19 virus. Many participants had already booked flights and hotels-all gone. What will happen to all the research insights currently written down in ICME and PME papers?In this editorial, we raise the question, What are the consequences for mathematics education and for research, now and in the future? Informal email conversation with colleagues across the globe (including China and Italy) points to many challenges and concerns that relate to life and to education. Most of the challenges are transdisciplinary, but some have unique characteristics for mathematics learning. There is research on mathematics learning at a distance through non-traditional media, but we expect that the consequences of this pandemic will inspire more such research. Many of our conversations these days turn to deep reflection in an attempt to stay calm and clear-headed. In this editorial, we summarize a few experiences that people have shared with us, hoping that proper study will help people formulate lessons for today and tomorrow. This is particularly challenging in times where change is so rapid. As educators and researchers, we find ourselves in a tension eloquently formulated by Søren Kierkegaard: "It is perfectly true, as the philosophers say, that life must be understood backwards. But they forget the other proposition, that it must be lived forwards"