“…Second, a number of mathematical notations to denote learning times of the three types of learning, lesson neighborhood and learner neighborhood are introduced in a detailed way. Third, a five-steps procedure, based on a number of hypotheses, learning times, lesson neighborhood, learner neighborhood, content filtering technique [38], collaborative filtering technique [7,8,26,28,32,37,54], similarity between two lessons and similarity between two learners, is introduced to estimate the value of EnablingTime(Lr, Ls). It denotes the amount of lesson learning-time for lesson Ls it takes to enable learner Lr to do all the exercises of Ls.…”