function order3
    hold on
    y=dsolve('D3y=2*(D2y-1)*cot(x)', 'Dy(pi/2)=0', 'D2y(pi/2)=1', 'y(pi/2)=1', 'x')
    x=-5:0.01:5;
    plot(x,eval(y),'r')

   y = eval(y)
   [m,im]=min(y)
   [M,iM]=max(y)
   plot(x(im), m, 'bo')
   plot(x(iM), M, 'go')

end