function [xdot] = poiseuille(sigma,x)
%
%  State-space model of [H6.9a]
%
global taum
gama = 10;
epsilon = 1e-10;
vhat = x(1);
w = x(2);
tau = x(3);
rho = 1 - sigma;
rho3 = rho*rho*rho;
tau2 = tau*tau;
taum1 = taum + 1;
taum2 = taum1*taum1;
taum3 = taum2*taum1;
vhatdot = sqrt(2*gama)*rho*tau2/taum2;
wdot = gama*rho3*tau2/taum3;
if rho > 0,
  taudot = -tau*w/rho;
 else
  taudot = 0;
end
xdot = zeros(3,1);
xdot(1) = vhatdot;
xdot(2) = wdot;
xdot(3) = taudot;
%
return