Hint on Eq.(7) in the paper

[minyez]

Hi Alexandr,

Thank you for creating this project. I was reading Paper-03-18-2013 about the theory, implementation and tests.

For the numerical differentiation, I managed to find Eq.(6) for the second-order derivative w.r.t. x in the Abramowitz and Stegun's book (1972, p884, 25.3.24), but failed for the mixed one, i.e. Eq.(7). Could you give some hint on its source?

Thanks!

Cheers
Minye