safeUroot | R Documentation |
safeUroot()
as a “safe” version of
uniroot()
searches for a root (that is, zero) of the
function f
with respect to its first argument.
“Safe” means searching for the correct interval
= c(lower,upper)
if sign(f(x))
does not satisfy the
requirements at the interval end points; see the ‘Details’
section.
safeUroot(f, interval, ...,
lower = min(interval), upper = max(interval),
f.lower = f(lower, ...), f.upper = f(upper, ...),
Sig = NULL, check.conv = FALSE,
tol = .Machine$double.eps^0.25, maxiter = 1000, trace = 0)
f |
function |
interval |
interval |
... |
additional named or unnamed arguments to be passed
to |
lower , upper |
lower and upper endpoint of search interval |
f.lower , f.upper |
function value at |
Sig |
desired sign of |
check.conv |
logical indicating whether a convergence warning of the
underlying |
tol |
the desired accuracy, that is, convergence tolerance. |
maxiter |
maximal number of iterations |
trace |
number determining tracing |
If it is known how f
changes sign at the root
x_0
, that is, if the function is increasing or decreasing there,
Sig
can be specified, typically as S := \pm 1
,
to require S = \mathrm{sign}(f(x_0 + \epsilon))
at the solution. In that case, the search interval [l,u]
must
be such that S * f(l) <= 0
and S * f(u) >= 0
.
Otherwise, by default, when Sig=NULL
, the search interval
[l,u]
must satisfy f(l) * f(u) <= 0
.
In both cases, when the requirement is not satisfied, safeUroot()
tries to enlarge the interval until the requirement is satisfied.
A list with four components, root
, f.root
, iter
and estim.prec
; see uniroot
.
uniroot
.
f1 <- function(x) (121 - x^2)/(x^2+1)
f2 <- function(x) exp(-x)*(x - 12)
try(uniroot(f1, c(0,10)))
try(uniroot(f2, c(0,2)))
##--> error: f() .. end points not of opposite sign
## where as safeUroot() simply first enlarges the search interval:
safeUroot(f1, c(0,10),trace=1)
safeUroot(f2, c(0,2), trace=2)
## no way to find a zero of a positive function:
try( safeUroot(exp, c(0,2), trace=TRUE) )
## Convergence checking :
safeUroot(sinc, c(0,5), maxiter=4) #-> "just" a warning
try( # an error, now with check.conv=TRUE
safeUroot(sinc, c(0,5), maxiter=4, check.conv=TRUE) )
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