Minimum number of edges in graphs that are both P2- and Pi-connected. - Yoko Usami (Discret. Math., 1983)

Extremal graphs of diameter at most 6 after deleting any vertex. - Yoko Usami (J. Graph Theory, 1985)

Critically (k, k)-connected graphs. - Kiyoshi Ando; Yoko Usami (Discret. Math., 1987)