[R] numerical derivative

[Z P]

>Dear all,
>
>I now want to get the numerical derivative of some multivariate function 
>y=f(x_1,...,x_k) at some specific point x_0=(x_10,...,x_k0).
>
>I know deriv() funtion can give the numerical derivative when f is an known 
>fuction. Now the f is an unknown fuction, but I can give many points 
>x_i=(x_1i,...,x_ki) around x_0 and their respective response y_i. I just 
>want some rough estimation of this derivative, so I do not want to do the 
>non-parametric regression. It is better to have an existing function in R 
>to do the job like derive() for the known function f. Thank you.
>
>Regards,
>
>Zhen
>