For ten points, identify this French-Italian author of M?anique analytique.
His early work on isoperimetrical problems impressed Euler and his theorem states that the order of a subgroup of a finite group is a factor of the group's order.  The calculus of variations is a key part of his geometry-free explanation of mechanics based on the function of kinetic and potential energy bearing his name.  For ten points, identify this French-Italian author of M?anique analytique.
His early work on isoperimetrical problems impressed Euler and his theorem states that the order of a subgroup of a finite group is a factor of the group's order.  
Joseph Louis Lagrange