  The exact canonical partition function of a hard disk system in a narrow
quasi-one dimensional pore of given length and width is derived analytically in
the thermodynamic limit. As a result the many body problem is reduced to
solving two transcendental equations which can be easily done numerically. The
longitudinal and transverse pressures in the whole density range are presented
for three different pore widths. The transition from the solidlike zigzag to
the liquidlike state is found to be quite sharp in the density scale but shows
no genuine singularity. This transition is quantitatively described by the
distribution of zigzag's windows through which disks exchange their positions
across the pore.
