1. Arhangel'skii, A.V.: The structure and classification of topological spaces and cardinal invariants. Russian Math. Surveys 33, 33–96 (1978)
2. Bella, A.: On a theorem of Šapirovskiǐ on the size of a sequential space. Math. Pannon. 16, 173–177 (2005)
3. Basile, D., Bella, A., Ridderbos, G.J.: Weak extent, submetrizability and diagonal degrees. Houston J. Math. 40, 255–266 (2014)
4. R. Engelking, General Topology, PWN (Warsaw, 1977)
5. Gartside, P.M.: Cardinal invariants of monotonically normal spaces. Topology Appl. 77, 303–314 (1997)