2002 年 78 巻 10 号 p. 1082-1083
To develop a statistical theory, one has to define a probability density on the phase space of ensemble. The measure of the space, then, must be invariant against the map (flow) representing the dynamics. This property is warranted by the Liouville theorem in the canonical Hamitonian dynamics. For some infinite dimension dynamics, we can construct an ”invariant measure” by an appropriate eigenfunction expansion of fields, which provides a basis to construct statistical mechanical theory of the fields.