1st Derivative, 5 point stencil[3]:
2nd Derivative, 5 point rule[1]
Mixed Derivative Formula[2]
[1] http://www.sitmo.com/article/numerical-differentiation/
[2] Numerical Recipes in C
[3] Wikipedia. "Five Point Stencil." http://en.wikipedia.org/wiki/Five-point_stencil
[4] http://www.holoborodko.com/pavel/numerical-methods/