I'm following this thread and got curious about Ryacas. After installing 
yacas and Ryacas, I was alerted to the missing XML package for Windows. 
I couldn't find in on a couple of mirrors so I tried 
http://www.omegahat.org/RSXML/ and

install.packages("XML", repos = "http://www.omegahat.org/R")

but got

Installing package(s) into 'D:/Users/David Stevens/Documents/R/win-library/2.14'
(as 'lib' is unspecified)
Warning message:
In getDependencies(pkgs, dependencies, available, lib) :
   package 'XML' is not available (for R version 2.14.1)

for my troubles. The date on the omegahat site is 17 Jan 2012. Am I missing something?

David Stevens



On 1/22/2012 12:39 PM, Gabor Grothendieck wrote:
> On Sun, Jan 22, 2012 at 1:25 PM, Eliano<eliano.m.marques@gmail.com>  wrote:
>> People,
>>
>> I'm researching some Bayesian statistic topics and in the midle of my study
>> i found a very simple problem and i'm trying to find a simple package to
>> solve this type of equations:
>>
>> Lets say that i need to compute beta values for the beta distribution and i
>> now for example:
>>
>> E(teta)=a/(a+b) = 0,5
>> Var(teta)=ab/((a+b)^2(a+b+1))=0.05
>>
>> So if i want to solve this to non-linear system to find a,b for the beta
>> distribution wich pack should i use?
>>
> The equations giving a and b as a function of m and v are:
>
>> library(Ryacas)
>> a<- Sym("a"); b<- Sym("b")
>> m<- Sym("m"); v<- Sym("v")
>> Solve( List(a/(a+b) == m, a*b/((a+b)^2*(a+b+1)) == v), List(a, b) )
> expression(list(list(a == m^2 * (1 - m)/v - m, b == a/m - a)))
>
> Based on the above we write this R function:
>
> beta.parms<- function(m, v) {
>     a<- m^2 * (1-m)/v - m
>     b<- a/m - a
>     c(a = a, b = b)
> }
>
> and run it:
>
>> beta.parms(m = 0.5, v = 0.05)
> a b
> 2 2
>

-- 
David K Stevens, P.E., Ph.D., Professor
Civil and Environmental Engineering
Utah Water Research Laboratory
8200 Old Main Hill
Logan, UT  84322-8200
435 797 3229 - voice
435 797 1363 - fax
david.stevens@usu.edu




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