Abstract Several important results for finite posets are proved. With these results and the specialization order of a topology, as well as relationships between topologies and orderings, on a 4-element set the total number of $T_0 $-topologies to be 219 and the total number of topologies to be 355 are obtained.
RONG Yu-yin, XU Luo-shan, XIE Li-na. Finite posets and calculating of the total number of $T_0$-topologies. Applied Mathematics A Journal of Chinese Universities, 2015, 30(4): 457-461.