Description Usage Arguments Details Value Source References See Also Examples

The function `unirootR`

searches the interval from `lower`

to `upper`

for a root (i.e., zero) of the function `f`

with
respect to its first argument.

`unirootR()`

is “clone” of `uniroot()`

,
written entirely in **R**, in a way that it works with
`mpfr`

-numbers as well.

1 2 3 4 5 6 |

`f` |
the function for which the root is sought. |

`interval` |
a vector containing the end-points of the interval to be searched for the root. |

`...` |
additional named or unnamed arguments to be passed
to |

`lower, upper` |
the lower and upper end points of the interval to be searched. |

`f.lower, f.upper` |
the same as |

`verbose` |
logical (or integer) indicating if (and how much) verbose output should be produced during the iterations. |

`tol` |
the desired accuracy (convergence tolerance). |

`maxiter` |
the maximum number of iterations. |

`epsC` |
positive number or The default will set this to This is factually a lower bound for the achievable lower bound, and
hence, setting |

Note that arguments after `...`

must be matched exactly.

Either `interval`

or both `lower`

and `upper`

must be
specified: the upper endpoint must be strictly larger than the lower
endpoint. The function values at the endpoints must be of opposite
signs (or zero).

The function only uses **R** code with basic arithmetic, such that it
should also work with “generalized” numbers (such as
`mpfr`

-numbers) as long the necessary
`Ops`

methods are defined for those.

The underlying algorithm assumes a continuous function (which then is known to have at least one root in the interval).

Convergence is declared either if `f(x) == 0`

or the change in
`x`

for one step of the algorithm is less than `tol`

(plus an
allowance for representation error in `x`

).

If the algorithm does not converge in `maxiter`

steps, a warning
is printed and the current approximation is returned.

`f`

will be called as `f(`

for a (generalized)
numeric value of `x`, ...)`x`.

A list with four components: `root`

and `f.root`

give the
location of the root and the value of the function evaluated at that
point. `iter`

and `estim.prec`

give the number of iterations
used and an approximate estimated precision for `root`

. (If the
root occurs at one of the endpoints, the estimated precision is
`NA`

.)

Based on `zeroin()`

(in package rootoned) by John Nash who
manually translated the C code in **R**'s `zeroin.c`

and on
`uniroot()`

in **R**'s sources.

Brent, R. (1973), see `uniroot`

.

`polyroot`

for all complex roots of a polynomial;
`optimize`

, `nlm`

.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 | ```
require(utils) # for str
## some platforms hit zero exactly on the first step:
## if so the estimated precision is 2/3.
f <- function (x,a) x - a
str(xmin <- unirootR(f, c(0, 1), tol = 0.0001, a = 1/3))
## handheld calculator example: fixpoint of cos(.):
rc <- unirootR(function(x) cos(x) - x, lower=-pi, upper=pi, tol = 1e-9)
rc$root
## the same with much higher precision:
rcM <- unirootR(function(x) cos(x) - x,
interval= mpfr(c(-3,3), 300), tol = 1e-40)
rcM
x0 <- rcM$root
stopifnot(all.equal(cos(x0), x0,
tol = 1e-40))## 40 digits accurate!
str(unirootR(function(x) x*(x^2-1) + .5, lower = -2, upper = 2,
tol = 0.0001), digits.d = 10)
str(unirootR(function(x) x*(x^2-1) + .5, lower = -2, upper = 2,
tol = 1e-10 ), digits.d = 10)
## A sign change of f(.), but not a zero but rather a "pole":
tan. <- function(x) tan(x * (Const("pi",200)/180))# == tan( <angle> )
(rtan <- unirootR(tan., interval = mpfr(c(80,100), 200), tol = 1e-40))
## finds 90 {"ok"}, and now gives a warning
## Find the smallest value x for which exp(x) > 0 (numerically):
r <- unirootR(function(x) 1e80*exp(x)-1e-300, c(-1000,0), tol = 1e-15)
str(r, digits.d = 15) ##> around -745, depending on the platform.
exp(r$root) # = 0, but not for r$root * 0.999...
minexp <- r$root * (1 - 10*.Machine$double.eps)
exp(minexp) # typically denormalized
## --- using mpfr-numbers :
## Find the smallest value x for which exp(x) > 0 ("numerically");
## Note that mpfr-numbers underflow *MUCH* later than doubles:
## one of the smallest mpfr-numbers {see also ?mpfr-class } :
(ep.M <- mpfr(2, 55) ^ - ((2^30 + 1) * (1 - 1e-15)))
r <- unirootR(function(x) 1e99* exp(x) - ep.M, mpfr(c(-1e20, 0), 200))
r # 97 iterations; f.root is very similar to ep.M
``` |

Rmpfr documentation built on Jan. 13, 2018, 1:11 a.m.

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