Now the projection of the force onto the direction of motion
is given by the integral (7.29)
In the following we will introduce a two-dimensional density
. The
square of the unperturbed homogeneous solution equals to it
. The
integral (7.32) is different from zero only if integrand has poles, which
means that the velocity
must be larger than the speed of sound
. Only momenta
smaller than
(see eq.(7.26)) contribute to the integral
![]() |
(7.33) |
We recall simple integral equality
and finally have
In a quasi two-dimensional system, i.e. when the gas is confined in the
-direction by the harmonic potential
, the two-dimensional
coupling constant equals (see 1.122)
![]() |
(7.35) |
Notice again that our calculations do not take into account creation of vortex pairs
which is possible at .