polrootsreal

Function: polrootsreal
Section: polynomials
C-Name: realroots
Prototype: GDGp
Description:
  (gen,?gen):vec:prec realroots($1, $2, $prec)
Help: polrootsreal(T, {ab}): real roots of the polynomial T with real
 coefficients, using Uspensky's method. In interval ab = [a,b] if present.
Doc: real roots of the polynomial $T$ with real coefficients, multiple
 roots being included according to their multiplicity. If the polynomial
 does not have rational coefficients, it is first rescaled and rounded.
 The roots are given to a relative accuracy of \kbd{realprecision}.
 If argument \var{ab} is
 present, it must be a vector $[a,b]$ with two components (of type
 \typ{INT}, \typ{FRAC} or \typ{INFINITY}) and we restrict to roots belonging
 to that closed interval.
 \bprog
 ? \p9
 ? polrootsreal(x^2-2)
 %1 = [-1.41421356, 1.41421356]~
 ? polrootsreal(x^2-2, [1,+oo])
 %2 = [1.41421356]~
 ? polrootsreal(x^2-2, [2,3])
 %3 = []~
 ? polrootsreal((x-1)*(x-2), [2,3])
 %4 = [2.00000000]~
 @eprog\noindent
 The algorithm used is a modification of Uspensky's method (relying on
 Descartes's rule of sign), following Rouillier and Zimmerman's article
 ``Efficient isolation of a polynomial real roots''
 (\url{http://hal.inria.fr/inria-00072518/}). Barring bugs, it is guaranteed
 to converge and to give the roots to the required accuracy.

 \misctitle{Remark} If the polynomial $T$ is of the
 form $Q(x^h)$ for some $h\geq 2$ and \var{ab} is omitted, the routine will
 apply the algorithm to $Q$ (restricting to nonnegative roots when $h$ is
 even), then take $h$-th roots. On the other hand, if you want to specify
 \var{ab}, you should apply the routine to $Q$ yourself and a suitable
 interval $[a',b']$ using approximate $h$-th roots adapted to your problem:
 the function will not perform this change of variables if \var{ab} is present.