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After each slide upload (or creation) there is a upload/creation notification with a topic indication (e.g. maths, chemistry...). Users with a given mobile app specify their (topic) interests. They with will receive a notification for every matching.
During a slideshow, the people watching/attending (audience) can provide annotations that could be added as notifications. Similar to the comments in a web page. Students can see the comments for a given slide from other year students. The system can show how many people is watching that specific slide. Students could talk about a specific slide. When the teacher modifies a given slide, present (¿and previous?) people is notified about the changes.
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