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From the website: "Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook™ textbooks). The Site is designed to serve as both an information resource and an application platform for K-12 teachers, educators and students who wish to join our online community."
From the National Radio Astronomy Observatory website: "Essential Radio Astronomy (ERA) is a one-semester course intended for astronomy graduate students and advanced undergraduates with backgrounds in astronomy, physics, or engineering. The goal of ERA is fostering the community of researchers using radio astronomy by attracting and training the most talented university students."
From the website: "Open Yale Courses provides free and open access to a selection of introductory courses taught by distinguished teachers and scholars at Yale University. The aim of the project is to expand access to educational materials for all who wish to learn."