8Albert D. Knowledge Structure [M]. New York. Springer Verlag, 1994
9Albert D, Hockmeyer C. Adaptive and dynamic hypertext tutoring systems based on knowledge space theory [M]. In: du Boulay B, Mizoguchi R, eds. Artificial Intelligence in Education:Knowledge and Media in Learning Systems. vo 39 of Frontiers in Artficial Intelligence and Applications, Amsterdam, IOS Press, 1997. 553-555
10Stotts P D, Furuta R. Petri Net-based Hypertext Document Structure with Browsing Semantics [J]. ACM Transactions on Information System, 1989,7(1) : 3-29