Question: collect and sort

Hi all,

> x^2*(diff(y(x), x, x))+x*(diff(y(x), x))+(x^2-1/4)*y(x) = 0;

                 /  2      \                                   
               2 | d       |     / d      \   / 2   1\         
              x  |---- y(x)| + x |--- y(x)| + |x  - -| y(x) = 0
                 |   2     |     \ dx     /   \     4/         
                 \ dx      /                                   
> expand(1/x^2*(x^2*diff(diff(y(x), x),x)+x*(diff(y(x), x))+(x^2-1/4)*y(x) = 0));

                                 d                        
                  /  2      \   --- y(x)                  
                  | d       |    dx               y(x)    
                  |---- y(x)| + -------- + y(x) - ---- = 0
                  |   2     |      x                 2    
                  \ dx      /                     4 x     
> collect(diff(diff(y(x), x), x)+(diff(y(x), x))/x+y(x)-(1/4)*y(x)/x^2 = 0, y(x));

                                                  d          
                                   /  2      \   --- y(x)    
               /   1      \        | d       |    dx         
               |- ---- + 1| y(x) + |---- y(x)| + -------- = 0
               |     2    |        |   2     |      x        
               \  4 x     /        \ dx      /               
> sort((-(1/4)/x^2+1)*y(x)+diff(diff(y(x), x), x)+(diff(y(x), x))/x = 0);

                                                  d          
                                   /  2      \   --- y(x)    
               /   1      \        | d       |    dx         
               |- ---- + 1| y(x) + |---- y(x)| + -------- = 0
               |     2    |        |   2     |      x        
               \  4 x     /        \ dx      /               

of course, my goal is to order it starting with the second derivative.

Thanks for any help!

Mario