Berend Hasselman wrote:
>
>
> B. Jonathan B. Jonathan wrote:
>>
>> Hi there, I have following equations to be solved for a and b:
>>
>> a/(a+b) = x1
>> ab/((a+b)^2 (a+b+1)) = x2
>>
>> Is there any direct function available to solve them without
>> disentangling them manually? Thanks for your help.
>>
>
> There is a package nleqslv that will solve a system of equations with
> either a Newton or Broyden method.
>
> You can do this
>
> library(nleqslv)
>
> # trial values
>
> x1 <- .6
> x2 <- .2
>
> eqn <- function(par) {
> a <- par[1]
> b <- par[2]
> f <- numeric(2)
>
> f[1] <- a/(a+b) - x1
> f[2] <- a*b/((a+b)^2 * (a+b+1)) - x2
>
> f
> }
>
> pstart <- c(1,1)
>
> nleqslv(pstart,eqn,control=list(trace=1))
>
> #or this if you don't need the trace
>
> nleqslv(pstart,eqn)
>
> There is also a package BB that uses different methods and is especially
> geared towards very large systems.
>
> Berend
>
You can also pass values for x1 and x2 to your function like this
(you can replace the NA's with your default values).
eqn <- function(par,x1=NA,x2=NA) {
a <- par[1]
b <- par[2]
f <- numeric(2)
f[1] <- a/(a+b) - x1
f[2] <- a*b/((a+b)^2 * (a+b+1)) - x2
f
}
pstart <- c(1,1)
nleqslv(pstart,eqn, x1=.6, x2=.2)
Berend
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