On Tue, Jun 28, 2011 at 10:03 PM, Lisa <lisa...@gmail.com> wrote: > Dear all, > > I just want to get the derivative of a function that looks like: > > y = exp(x1*b) / (exp(x1*b) + exp(x2*b)) > > where y is a scalar, x1, x2, and b are vectors. I am going to take the > derivative of b with respect to y, but I cannot derive an expression in > which b is function of y. I know there is another way to get the similar > result, i.e., first take the derivative of y with respect to each element of > b, and then take its reciprocal. But it is not what I want. Could someone > please tell me how to solve this problem? Thank you in advance.
Assuming you meant the derivative of y with respect to b: > D(expression(exp(x1*b) / (exp(x1*b) + exp(x2*b))), "b") exp(x1 * b) * x1/(exp(x1 * b) + exp(x2 * b)) - exp(x1 * b) * (exp(x1 * b) * x1 + exp(x2 * b) * x2)/(exp(x1 * b) + exp(x2 * b))^2 See ?D and also note deriv on the same help page for another alternative. -- Statistics & Software Consulting GKX Group, GKX Associates Inc. tel: 1-877-GKX-GROUP email: ggrothendieck at gmail.com ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.