Let's take a system, for simplicity with only one degree of freedom, described by a certain lagrangian
L[x,\dot x]
I define the momentum
p=\frac{\partial L}{\partial\dot x}
Now, if I make a change of coordinates
x\longmapsto y\qquad\qquad\qquad(1)
I obtain a second lagrangian
M[y,\dot...