I have a discrete signal (vector of numbers) coming from a measurement. This signal has been filtered so that the noise has been removed. Now I am looking for an analytical representation of the signal (don't know if it is correct expression, I mean an analytical function that approximates the signal). My idea is to define an interpolation maybe using a spline. Any idea of how I can extract nodes from the vector to define a spline with a specific approximation? Can I consider the second (numerical) derivative as an estimation of the oscillation?