diff function (second derivative)

Say that I have an array A on which I apply the diff function in order to approximate the second derivative. After looking at the results I want to locate the values in A which generated a certain value in ddA. How would one do that?

A =

      0.6715
      1.2075
      0.7172
      1.6302
      0.4889
      1.0347
      0.7269
      0.3034
      0.2939
      0.7873

ddA=diff(A,2)

ddA =

     -1.0262
      1.4032
     -2.0543    <--- This value is interesting
      1.6871
     -0.8536
     -0.1156
      0.4139
      0.5030

How do I find the values in A which generated the interesting value in ddA?

Answer 1

[Walter Roberson]

The N'th value of diff(A,2) is

A(N+2)-2*A(N+1)+A(N)

Note: the value of ddA that you show does not correspond to the value of A that you show.

[jakob ekwall]

Thanks!

They should correspond now.