It's the second symmetric derivative. https://en.wikipedia.org/wiki/Symmetric_derivative#The_second_symmetric_derivative https://en.wikipedia.org/wiki/Symmetri

It's the second symmetric derivative. https://en.wikipedia.org/wiki/Symmetric_derivative#The_second_symmetric_derivative https://en.wikipedia.org/wiki/Symmetric_derivative#The_secon... https://en.wikipedia.org/wiki/Second_derivative#Limit https://en.wikipedia.org/wiki/Second_derivative#Limit > The limit is called the second symmetric derivative. Note that the second symmetric derivative may exist even when the (usual) second derivative does not. > This limit can be viewed as a continuous version of the second difference for sequences. So it actually can do what I intended it to do, nothing original - but there was a degree of satisfaction in deriving it myself.