J. Zuo et Yx. Gui, ETA-XI SPACETIME AND THE TILDE FREEDOM IN THERMO FIELD-DYNAMICS, Journal of physics. A, mathematical and general, 28(17), 1995, pp. 4907-4913
The CM operations are introduced in the theory of eta-xi spacetime. Th
ey have the same properties as the tilde rules in thermo field dynamic
s (TFD); we can, therefore, identify them with the tilde operation. Th
is leads to the conclusion that the tilde system in TFD can be regarde
d as the system in region II (mirror universe) of eta-xi spacetime.