1. V. Hurewicz and H. Wallman, Dimension theory, Revised edition, Princeton University Press, Princeton, NJ, 1996.
2. L. Lyusternik and L. Schnirelmann, Topological methods in variational problems, Issledowatelskiy Institut Mathematiki i Mechaniki pri M G U, Moscow, 1930 (Russian).
3. E. Spanier, Algebraic topology, McGraw-Hill, NewYork, 1966.
4. M. Arkowitz and J. Strom, The sectional category of a map, Proc. of Royal Soc. Edinburgh, 134A (2004), 639–650.
5. I. James, On category in the sense of Lusternik–Schnirelmann, Topology 17 (1978), 331–348.