Numerical differentiation:

• order (1, 2, ...)
• type (centered, forward, backward)
• accuracy (O(h), O(h^2), ..)

Links:
http://websrv.cs.umt.edu/isis/index.php/Finite_differencing:_Introduction
http://scicomp.stackexchange.com/questions/480/how-can-i-numerically-differentiate-an-unevenly-sampled-function
https://mathformeremortals.wordpress.com/2013/01/12/a-numerical-second-derivative-from-three-points/
http://fundamentalthinking.blogspot.de/2014/08/second-order-finite-difference-schemes.html
http://www.scientificpython.net/pyblog/non-uniform-first-order-finite-differences
http://www.uobabylon.edu.iq/uobcoleges/ad_downloads/4_27512_277.pdf
http://www.m-hikari.com/ijma/ijma-password-2009/ijma-password17-20-2009/bhadauriaIJMA17-20-2009.pdf
http://www.lboro.ac.uk/microsites/maths/research/preprints/papers04/04-35.pdf
http://amath.colorado.edu/faculty/fornberg/Docs/MathComp_88_FD_formulas.pdf
http://royalsocietypublishing.org/content/royprsa/461/2059/1975.full.pdf
http://www.originlab.com/doc/Origin-Help/Math-Differentiate