Find Inflection Points from excel file

Hi,

I have an excel file with a random 100 numbers, and i`m trying to find the Inflection Points. In the X axis its 1-100 (even jumps).

I saw that using 2nd derivative is the way (when it equals 0), all i need is just to do y2-y1/x2-x1 in the excel? Because i tried that and the 2nd derivative was never 0 (was very close).

[Jake G]

are these 100 random numbers coming from a function that you know will give you inflection points? you could try recreating the function with polyfit or something than solving for the second derivative.

Answer 1

[Roger Stafford]

For your situation with discrete points, I would look for places where the curvature changes sign. With four successive points, compare the curvature of the first three with that of the last three and if their signs differ, there is a point of inflection somewhere in the neighborhood. That is about the most you can say with such data. If your data is truly random, a search for inflection points would be meaningless.

If (x1,y1), (x2,,y2), and (x3,y3) are three successive points, the sign of their curvature is given by the following:

   sign((x3-x2)*(y1-y2)-(x1-x2)*(y3-y2))