`geex`

`rootFUN`

By default, `geex`

uses the `rootSolve::multiroot`

function for finding roots of a set of estimating equations when `compute_roots = TRUE`

in `m_estimate()`

. However, a user can choose a different root find find algorithm via the `root_control`

argument.

For example, consider the following `estFUN`

which is Huber’s estimator for the center of symmetric distributions [@stefanski2002; example 6]. This example was chosen because it has a single root, so that the `stats::uniroot`

function can be used to find the roots.

```
myefun <- function(data, k = 1.5){
function(theta){
x <- data$Y1 - theta[1]
if(abs(x) <= k) x else sign(x) * k
}
}
```

Internally, `estFUN`

is used to build \(G_m = \sum_{i = 1}^m \psi(O_i, \theta)\) or in psuedo-code `f = sum(inner_estFUN(theta))`

. `f`

is passed to the root finding function along with options in the `root_control`

arguments. For example, `multiroot`

requires `f`

and `start`

(starting values for the algorithm:

```
multiroot_results <- m_estimate(
estFUN = myefun,
data = geexex,
root_control = setup_root_control(start = 3))
```

The `stats::uniroot`

function, however, requires the arguments `f`

and `interval`

(or `lower`

and `upper`

)

```
uniroot_results <- m_estimate(
estFUN = myefun,
data = geexex,
root_control = setup_root_control(stats::uniroot, interval = c(0, 10)))
```

Comparing results:

`roots(multiroot_results) - roots(uniroot_results)`

`## [1] 6.244845e-07`

They are basically the same, but this may not be true depending `f`

and the options given to the root finder.

All that is necessary for `rootFUN`

is a function where:

- the
*first*argument is the function whose roots are sought - the output is a named list where the root estimates can be identified by the
`rootFUN_object`

argument in`m_estimate`

. For example, both`uniroot`

and`multiroot`

return a list where the root estimates are in the item named “roots”. The default is`rootFUN_object = 'roots'`

, so this option works for both`uniroot`

and`multiroot`

.