Equivalence groupoids of classes of linear ordinary differential   equations and their group classification
  Admissible point transformations of classes of $r$th order linear ordinary differential equations (in particular, the whole class of such equations and its subclasses of equations in the rational form, the Laguerre-Forsyth form, the first and second Arnold forms) are exhaustively described. Using these results, the group classification of such equations is revisited within the algebraic approach in three different ways. 
Vyacheslav M. Boyko, Roman O. Popovych and Nataliya M. Shapoval
math.CA