Introduction to the Special Issue "Real Numbers and Computers"
This special issue contains a selection of papers presented during the
international conference "Real Numbers and Computers",
SaintEtienne, France, April 1995.
Efficient handling of real
numbers in a computer is not yet solved in a satisfying way,
yet. Although the "floatingpoint" formats most often used in
scientific computing usually give sufficient results, some
reliability problems may occur. Program portability could imply high
rewriting costs: some programs which work well with a machine, may
become unreliable with another one. Users (from computer algebra,
computational geometry,...) may need results far more accurate than
the ones obtained with usual number systems, if not "exact"
results.
Many members of the scientific community are concerned by
this problem, they could share their knowledge and come up with new
solutions. But they do not have the opportunity to meet, they do not
belong to the same scientific fields (computer science, number
theory, numerical analysis, computer algebra...) and they have a
different vocabulary. The aim of the SaintEtienne Conference was to
bring them together during this meeting, to establish some
collaborations.
The very first problem with the manipulation of real numbers in
computers is that the set of real numbers is not enumerable. As a
consequence, it is not possible to represent each real number by a
finite string of symbols taken from af inite alphabet. Depending on
the application, one has to choose which finite or enumerable subset
of the real numbers will be manipulated.
Even with enumerable subsets
of the reals, there remain serious problems: probably the most
important is that one cannot determine whether two computable real
numbers* are equal.
Let us now examine some problems related to the
discrete machine approximation of the continuous reals.

In computational geometry, the main problem is to construct
topologically consistent objects using (nonindependent) numerical
tests (e.g., signs of determinants). For instance, if we try to
compute the distance (a priori null) between the intersection point
of two straight lines and the straight lines separately, using
floatingpoint arithmetic, it is almost certain that the answers will
both be different and most likely nonnull. For many such situations
there is no obvious general treatment known.

A report of the United
States General Accounting Office (B247094, Feb. 1992), explains that
on February 1991 (during the war in the Gulf), a Patriot missile
defense system failed to intercept an incoming Scud, that killed 28
people, due to an inaccurate tracking calculation.
* A real
number x is computable if there is a machine that computes, for any
given integer n, a rational number that approximates x within error
(for instance, see KerI Ko, Complexity Theory of Real Functions,
Birkhauser, 1991).
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 If we try to compute the sequence defined as
using any boundedprecision arithmetic (such as floatingpoint
arithmetic) on any computer, then 100 will seem to be the limit value
of the sequence, while the correct limit value is 6.
 Define a sequence as
depending on your computer, the apparent limit value of will be 1,
2, 3 or 4.
Those are archetypes of problems that happen in realworld
computations (maybe especially during iterative calculations).
Many solutions have been proposed to cope with such problems:
 First, one may try to make the usual floating point arithmetic more
reliable, and to entirely specify it, in order to be able to
elaborate proofs and algorithms that use the specifications. For
instance, the IEEE754 and IEEE854 standards for floating point
computations** considerably helped to improve the quality and
portability of programs, and to design multiple precision or interval
arithmetic programs. The paper by Evgenija Popova (On a Formally
Correct Implementation ofIEEE Computer Arithmetic) is devoted to this
topic. The specification of the arithmetic may also help to get a
priori bounds on numerical errors for various computations. Raymond
Pavec (Some Algorithms Providing Rigorous Bounds for the Eigenvalues
of a Matrix) and Fabienne Jezequel (RoundOff Error Propagation in
the Solution of the Heath Equation by Finite Differences) obtained
such bounds.
 It is not always possible to get realistic bounds on
the numerical errors before the execution of a program, therefore it
is most desirable to build tools that dynamically compute such
bounds. Two possible ways to do this are the interval arithmetic,
illustrated by the paper by Svetoslav Markov (On Directed Interval
Arithmetic and its Applications) and the perturbation methods,
illustrated by the paper by Jalil Asserhine, JeanMarie Chesneaux and
JeanLuc Lamotte (Estimation of Round Off Errors on Several Computer Architectures).
 A more drastic solution is to get rid of the
usual floatingpoint arithmetic, and to build systems capable of
computing with arbitrary accuracy. One may try to represent real
numbers by flows of digits, as in online arithmetic. This is
illustrated by Thomas Lynch and Michael Schulte in their
** IEEE Standard 7541985 for Binary FloatingPoint Arithmetic,
IEEE. Reprinted in
SIGPLAN 22, 2, pp. 925. People interested by this topic should read
the paper by David Goldberg, What Every Computer Scientist Should
Know About Computer Arithmetic, ACM Computing Surveys, Vol. 23 No 1,
pp. 548
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paper (HighRadix OnLine Arithmetic for Credible and Accurate General
Purpose Computing). OnLine arithmetic was pioneered by one of the
invited speakers at the Conference, Milos Ercegovac, professor at the
University of California at Los Angeles. Another more general scheme
is to represent a number by flows of coefficients, such as those of a
continuedfraction expansion. This solution is explored by Peter
Kornerup and David W. Matula (LCF: A Lexicographic Binary
Representation of the Rationals, invited paper), by Asger Munk
Nielsen and Peter Kornerup (MSBFirst Digit Serial Arithmetic), and
by D. Lester (Exact Statistics and Continued Fractions). Peter
Kornerup, professor at Odense University, Denmark, was our second
invited speaker at the SaintEtienne Conference.
Other approaches, such as symbolic manipulation of numbers, are being
explored, but they are not represented in this special issue.
Approximating the continuous real arithmetic as closely as possible
with our inevitably discrete tools is attempting the impossible, but
it is fascinating. There are still many things to be done in this
domain, and we hope that the conference "Real Numbers and Computers
No 2", that will be held in Marseille, France, in April 1996, will
bring new solutions.
We would like to thank all the authors of submitted papers, including
those authors of papers that could not be included in this special
issue due to reviewer revision requests that could not be
accommodated in our tight time frame for publication. Special thanks
are due to the EditorinChief of the Journal of Universal Computer
Science, Hermann Maurer, for hosting this special issue.
JeanClaude BAJARD, Guest Editor
Laboratoire LMI, Universite de Provence
13453 Marseille Cedex 13, FRANCE
Dominique MICHELUCCI, Guest Editor
Ecole des Mines de SaintEtienne,SIMADE, 158 cours Fauriel
42023 SaintEtienne Cedex 2, FRANCE
JeanMichel MoREAU, Guest Editor
Ecole des Mines de SaintEtienne, SIMADE, 158 cours Fauriel
42023 SaintEtienne Cedex 2, FRANCE
JeanMichel MULLER, Guest Editor
CNRS, Laboratoire LIP, ENS Lyon, 46 Allee d'Italie
69364 Lyon Cedex 07, FRANCE
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