I don't see the problem, except that you might want to think about
what the error message is telling you.
A little exploration of your function always helps, too.
> ss <- seq(-2, 2, len = 100)
> plot(ss, fn(ss), type = "l")
> uniroot(fn, c(-1, 1))
Erreur dans uniroot(fn, c(-1, 1)) : f() values at end points not of
opposite sign
> fn(-1)
[1] 3.330833
> fn(0)
[1] -0.0025
> fn(1)
[1] 0.5857353
> uniroot(fn, c(-1, 0))
$root
[1] -0.6999466
$f.root
[1] -13118.83
$iter
[1] 18
$estim.prec
[1] 7.70751e-05
> uniroot(fn, c(0, 1))
$root
[1] 0.001760625
$f.root
[1] 8.86832e-06
$iter
[1] 3
$estim.prec
[1] 6.103516e-05
> This below is not solvable with uniroot to find "a":
> fn=function(a){
> b=(0.7/a)-a
> (1/(a+b+1))-0.0025
> }
> uniroot(fn,c(-500,500)) gives
> "Error in uniroot(fn, c(-500, 500)) : f() values at end points not of
> opposite sign"
>
> I read R-help posts and someone wrote a function:
> http://finzi.psych.upenn.edu/R/Rhelp02a/archive/92407.html
> but it is not very precise. Is there any '"standard" function in R
> that can
> solve this? thanks.
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