Sakai's main field is functional analysis and mathematical physics. His textbook published in the Springer series in C *-algebras and W *-algebras, in which W *-algebras as C *-algebras are introduced with a predual, is widely used. That fact the W *-algebras may be defined in this way is known as a theorem of Sakai[3] (cf. a theorem of Kadison-Sakai.)[4]
Works
C *-algebras and W *-algebras, Springer-Verlag 1971, Ergebnisse der Mathematik und ihrer Grenzgebiete, Volume 60, ISBN3-540-63633-1 (republished in 1998 in Classics in Mathematics)
Operator algebras in dynamical systems, the theory of unbounded derivations in C*-algebras, Cambridge University Press (1991), ISBN0-521-40096-1